Gene Expression | Biology | Visionlearning

<!DOCTYPE html> <html lang="en" dir="ltr"> <head>  <meta http-equiv="X-UA-Compatible" content="ie=edge"> <meta charset="utf-8"> <base href="https://www.visionlearning.com"> <title>Gene Expression | Biology | Visionlearning</title> <link rel="canonical" href="https://www.visionlearning.com/en/library/biology/2/gene-expression/214"> <meta name="description" content="Through a look at the devastating Tay-Sachs disease and other hereditary conditions, this module explores the connection between genes and enzymes. The role of dominance vs. recessivity is examined. The module traces developments in our understanding of gene expression, starting with a rediscovery of Mendel’s laws of inheritance and built upon by the pioneering work of later scientists. The module introduces the Central Dogma of molecular biology, which is the one-way process of using DNA to make RNA and RNA to make proteins."> <meta name="keywords" content="genes, gene expression, hereditary, dominance, recessivity"> <meta name="viewport" content="width=device-width, initial-scale=1.0, shrink-to-fit=no"> <meta name="msvalidate.01" content="D8E20F39AD48052260032E56DE409970"> <script type="application/ld+json"> { "@context": "http://schema.org/", "@type": "Article", "mainEntityOfPage": { "@type": "WebPage", "@id": "https://visionlearning.com/en/library/biology/2/gene-expression/214" }, "name": "Gene Expression", "headline": "Gene Expression: Enzymes and hereditary conditions", "author": [ { "@type": "Person", "name": "David Warmflash, MD" } , { "@type": "Person", "name": "Nathan H Lents, Ph.D." } , { "@type": "Person", "name": "Bonnie Denmark, M.A./M.S." }], "datePublished": "2015-08-06 15:27:13", "dateModified": "2017-02-12T08:30:00+05:00", "image": { "@type": "ImageObject", "url": "/img/library/moduleImages/featured_image_214-23061209063659.jpg", "width": 696, "height": 464 }, "publisher": { "@type": "Organization", "name": "Visionlearning, Inc.", "logo": { "@type": "ImageObject", "url": "http://visionlearning.com/images/logo.png", "width": 278, "height": 60 } }, "description": "Through a look at the devastating Tay-Sachs disease and other hereditary conditions, this module explores the connection between genes and enzymes. The role of dominance vs. recessivity is examined. The module traces developments in our understanding of gene expression, starting with a rediscovery of Mendel’s laws of inheritance and built upon by the pioneering work of later scientists. The module introduces the Central Dogma of molecular biology, which is the one-way process of using DNA to make RNA and RNA to make proteins.", "keywords": "genes, gene expression, hereditary, dominance, recessivity", "inLanguage": { "@type": "Language", "name": "English", "alternateName": "en" }, "copyrightHolder": { "@type": "Organization", "name": "Visionlearning, Inc." }, "copyrightYear": "2015"} </script> <meta property="og:url" content="https://visionlearning.com/en/library/biology/2/gene-expression/214"> <meta property="og:title" content="Gene Expression | Biology | Visionlearning" /> <meta property="og:type" content="website"> <meta property="og:site_name" content="Visionlearning"> <meta property="og:description" content="Through a look at the devastating Tay-Sachs disease and other hereditary conditions, this module explores the connection between genes and enzymes. The role of dominance vs. recessivity is examined. The module traces developments in our understanding of gene expression, starting with a rediscovery of Mendel’s laws of inheritance and built upon by the pioneering work of later scientists. The module introduces the Central Dogma of molecular biology, which is the one-way process of using DNA to make RNA and RNA to make proteins."> <meta property="og:image" content="https://visionlearning.com/images/logo.png"> <meta property="fb:admins" content="100000299664514"> <link rel="stylesheet" type="text/css" href="/css/visionlearning.css">  <link rel="stylesheet" type="text/css" href="/css/visionlearning-icons.css">  <link rel="preload" href="https://fonts.gstatic.com"> <link rel="preload" href="https://fonts.googleapis.com/css2?family=Open+Sans:ital,wght@0,400;0,700;1,400;1,700&family=Schoolbell&display=swap"> <style> textarea.myEditor { width: 90%; height: 350px; } </style> <script type="text/x-mathjax-config" src="/js/mathjax-config.js"></script> <script id="MathJax-script" async src="/js/mathjax/tex-svg.js"></script> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script> </head> 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aria-controls="acc-panel-environmental-science" aria-expanded="false"> <span class="accordion__button__label"> Environmental Science </span> </button> <div class="accordion__panel" id="acc-panel-environmental-science" data-accordion="panel" aria-labelledby="acc-button-environmental-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-ecology" data-accordion="button" aria-controls="acc-panel-ecology" aria-expanded="false"> <span class="accordion__button__label"> Ecology </span> </button> <div class="accordion__panel" id="acc-panel-ecology" data-accordion="panel" aria-labelledby="acc-button-ecology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/biodiversity-i/276">Biodiversity I</a></li> <li><a href="/en/library/environmental-science/61/biodiversity-ii/281">Biodiversity II</a></li> <li><a href="/en/library/environmental-science/61/ecosystem-services/279">Ecosystem Services</a></li> <li><a href="/en/library/environmental-science/61/population-biology/287">Population Biology</a></li> </ul> </div> <button class="accordion__button" id="acc-button-earth-cycles" data-accordion="button" aria-controls="acc-panel-earth-cycles" aria-expanded="false"> <span class="accordion__button__label"> Earth Cycles </span> </button> <div class="accordion__panel" id="acc-panel-earth-cycles" data-accordion="panel" aria-labelledby="acc-button-earth-cycles" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/the-nitrogen-cycle/98">The Nitrogen Cycle</a></li> <li><a href="/en/library/environmental-science/61/the-carbon-cycle/95">The Carbon Cycle</a></li> <li><a href="/en/library/environmental-science/61/the-phosphorus-cycle/197">The Phosphorus Cycle</a></li> </ul> </div> <button class="accordion__button" id="acc-button-scientific-research" data-accordion="button" aria-controls="acc-panel-scientific-research" aria-expanded="false"> <span class="accordion__button__label"> Scientific Research </span> </button> <div class="accordion__panel" id="acc-panel-scientific-research" data-accordion="panel" aria-labelledby="acc-button-scientific-research" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/collaborative-research-in-the-arctic-towards-understanding-climate-change/183">Collaborative Research in the Arctic Towards Understanding Climate Change</a></li> <li><a href="/en/library/environmental-science/61/atmospheric-chemistry-research-that-changed-global-policy/211">Atmospheric Chemistry Research that Changed Global Policy</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-general-science" data-accordion="button" aria-controls="acc-panel-general-science" aria-expanded="false"> <span class="accordion__button__label"> General Science </span> </button> <div class="accordion__panel" id="acc-panel-general-science" data-accordion="panel" aria-labelledby="acc-button-general-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-methods" data-accordion="button" aria-controls="acc-panel-methods" aria-expanded="false"> <span class="accordion__button__label"> Methods </span> </button> <div class="accordion__panel" id="acc-panel-methods" data-accordion="panel" aria-labelledby="acc-button-methods" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/the-scientific-method/45">The Scientific Method</a></li> </ul> </div> <button class="accordion__button" id="acc-button-measurement" data-accordion="button" aria-controls="acc-panel-measurement" aria-expanded="false"> <span class="accordion__button__label"> Measurement </span> </button> <div class="accordion__panel" id="acc-panel-measurement" data-accordion="panel" aria-labelledby="acc-button-measurement" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/the-metric-system/47">The Metric System</a></li> </ul> </div> <button class="accordion__button" id="acc-button-physical-properties" data-accordion="button" aria-controls="acc-panel-physical-properties" aria-expanded="false"> <span class="accordion__button__label"> Physical Properties </span> </button> <div class="accordion__panel" id="acc-panel-physical-properties" data-accordion="panel" aria-labelledby="acc-button-physical-properties" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/temperature/48">Temperature</a></li> <li><a href="/en/library/general-science/3/density-and-buoyancy/37">Density and Buoyancy</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-math-in-science" data-accordion="button" aria-controls="acc-panel-math-in-science" aria-expanded="false"> <span class="accordion__button__label"> Math in Science </span> </button> <div class="accordion__panel" id="acc-panel-math-in-science" data-accordion="panel" aria-labelledby="acc-button-math-in-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-equations" data-accordion="button" aria-controls="acc-panel-equations" aria-expanded="false"> <span class="accordion__button__label"> Equations </span> </button> <div class="accordion__panel" id="acc-panel-equations" data-accordion="panel" aria-labelledby="acc-button-equations" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/unit-conversion/144">Unit Conversion</a></li> <li><a href="/en/library/math-in-science/62/linear-equations/194">Linear Equations</a></li> <li><a href="/en/library/math-in-science/62/exponential-equations-i/206">Exponential Equations I</a></li> <li><a href="/en/library/math-in-science/62/exponential-equations-ii/210">Exponential Equations II</a></li> <li><a href="/en/library/math-in-science/62/scientific-notation/250">Scientific Notation</a></li> <li><a href="/en/library/math-in-science/62/measurement/257">Measurement</a></li> </ul> </div> <button class="accordion__button" id="acc-button-statistics" data-accordion="button" aria-controls="acc-panel-statistics" aria-expanded="false"> <span class="accordion__button__label"> Statistics </span> </button> <div class="accordion__panel" id="acc-panel-statistics" data-accordion="panel" aria-labelledby="acc-button-statistics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/introduction-to-descriptive-statistics/218">Introduction to Descriptive Statistics</a></li> <li><a href="/en/library/math-in-science/62/introduction-to-inferential-statistics/224">Introduction to Inferential Statistics</a></li> <li><a href="/en/library/math-in-science/62/statistical-techniques/239">Statistical Techniques</a></li> </ul> </div> <button class="accordion__button" id="acc-button-trigonometric-functions" data-accordion="button" aria-controls="acc-panel-trigonometric-functions" aria-expanded="false"> <span class="accordion__button__label"> Trigonometric Functions </span> </button> <div class="accordion__panel" id="acc-panel-trigonometric-functions" data-accordion="panel" aria-labelledby="acc-button-trigonometric-functions" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/wave-mathematics/131">Wave Mathematics</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-physics" data-accordion="button" aria-controls="acc-panel-physics" aria-expanded="false"> <span class="accordion__button__label"> Physics </span> </button> <div class="accordion__panel" id="acc-panel-physics" data-accordion="panel" aria-labelledby="acc-button-physics" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-light-and-optics" data-accordion="button" aria-controls="acc-panel-light-and-optics" aria-expanded="false"> <span class="accordion__button__label"> Light and Optics </span> </button> <div class="accordion__panel" id="acc-panel-light-and-optics" data-accordion="panel" aria-labelledby="acc-button-light-and-optics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/physics/24/the-nature-of-light/132">The Nature of Light</a></li> <li><a href="/en/library/physics/24/electromagnetism-and-light/138">Electromagnetism and Light</a></li> </ul> </div> <button class="accordion__button" id="acc-button-mechanics" data-accordion="button" aria-controls="acc-panel-mechanics" aria-expanded="false"> <span class="accordion__button__label"> Mechanics </span> </button> <div class="accordion__panel" id="acc-panel-mechanics" data-accordion="panel" aria-labelledby="acc-button-mechanics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/physics/24/defining-energy/199">Defining Energy</a></li> <li><a href="/en/library/physics/24/waves-and-wave-motion/102">Waves and Wave Motion</a></li> <li><a href="/en/library/physics/24/gravity/118">Gravity</a></li> <li><a href="/en/library/physics/24/thermodynamics-i/200">Thermodynamics I</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-process-of-science" data-accordion="button" aria-controls="acc-panel-process-of-science" aria-expanded="false"> <span class="accordion__button__label"> Process of Science </span> </button> <div class="accordion__panel" id="acc-panel-process-of-science" data-accordion="panel" aria-labelledby="acc-button-process-of-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-introduction" data-accordion="button" aria-controls="acc-panel-introduction" aria-expanded="false"> <span class="accordion__button__label"> Introduction </span> </button> <div class="accordion__panel" id="acc-panel-introduction" data-accordion="panel" aria-labelledby="acc-button-introduction" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-process-of-science/176">The Process of Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-the-culture-of-science" data-accordion="button" aria-controls="acc-panel-the-culture-of-science" aria-expanded="false"> <span class="accordion__button__label"> The Culture of Science </span> </button> <div class="accordion__panel" id="acc-panel-the-culture-of-science" data-accordion="panel" aria-labelledby="acc-button-the-culture-of-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-nature-of-scientific-knowledge/185">The Nature of Scientific Knowledge</a></li> <li><a href="/en/library/process-of-science/49/scientists-and-the-scientific-community/172">Scientists and the Scientific Community</a></li> <li><a href="/en/library/process-of-science/49/scientific-ethics/161">Scientific Ethics</a></li> <li><a href="/en/library/process-of-science/49/scientific-institutions-and-societies/162">Scientific Institutions and Societies</a></li> </ul> </div> <button class="accordion__button" id="acc-button-ideas-in-science" data-accordion="button" aria-controls="acc-panel-ideas-in-science" aria-expanded="false"> <span class="accordion__button__label"> Ideas in Science </span> </button> <div class="accordion__panel" id="acc-panel-ideas-in-science" data-accordion="panel" aria-labelledby="acc-button-ideas-in-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/theories-hypotheses-and-laws/177">Theories, Hypotheses, and Laws</a></li> <li><a href="/en/library/process-of-science/49/scientific-controversy/181">Scientific Controversy</a></li> <li><a href="/en/library/process-of-science/49/creativity-in-science/182">Creativity in Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-research-methods" data-accordion="button" aria-controls="acc-panel-research-methods" aria-expanded="false"> <span class="accordion__button__label"> Research Methods </span> </button> <div class="accordion__panel" id="acc-panel-research-methods" data-accordion="panel" aria-labelledby="acc-button-research-methods" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-practice-of-science/148">The Practice of Science</a></li> <li><a href="/en/library/process-of-science/49/experimentation-in-scientific-research/150">Experimentation in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/description-in-scientific-research/151">Description in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/comparison-in-scientific-research/152">Comparison in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/modeling-in-scientific-research/153">Modeling in Scientific Research</a></li> </ul> </div> <button class="accordion__button" id="acc-button-data" data-accordion="button" aria-controls="acc-panel-data" aria-expanded="false"> <span class="accordion__button__label"> Data </span> </button> <div class="accordion__panel" id="acc-panel-data" data-accordion="panel" aria-labelledby="acc-button-data" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/data-analysis-and-interpretation/154">Data Analysis and Interpretation</a></li> <li><a href="/en/library/process-of-science/49/uncertainty-error-and-confidence/157">Uncertainty, Error, and Confidence</a></li> <li><a href="/en/library/process-of-science/49/statistics-in-science/155">Statistics in Science</a></li> <li><a href="/en/library/process-of-science/49/using-graphs-and-visual-data-in-science/156">Using Graphs and Visual Data in Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-scientific-communication" data-accordion="button" aria-controls="acc-panel-scientific-communication" aria-expanded="false"> <span class="accordion__button__label"> Scientific Communication </span> </button> <div class="accordion__panel" id="acc-panel-scientific-communication" data-accordion="panel" aria-labelledby="acc-button-scientific-communication" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/understanding-scientific-journals-and-articles/158">Understanding Scientific Journals and Articles</a></li> <li><a href="/en/library/process-of-science/49/utilizing-the-scientific-literature/173">Utilizing the Scientific Literature</a></li> <li><a href="/en/library/process-of-science/49/peer-review-in-scientific-publishing/159">Peer Review in Scientific Publishing</a></li> <li><a href="/en/library/process-of-science/49/the-how-and-why-of-scientific-meetings/186">The How and Why of Scientific Meetings</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-scientists-and-research" data-accordion="button" aria-controls="acc-panel-scientists-and-research" aria-expanded="false"> <span class="accordion__button__label"> Scientists and Research </span> </button> <div class="accordion__panel" id="acc-panel-scientists-and-research" data-accordion="panel" aria-labelledby="acc-button-scientists-and-research" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-scientific-research" data-accordion="button" aria-controls="acc-panel-scientific-research" aria-expanded="false"> <span class="accordion__button__label"> Scientific Research </span> </button> <div class="accordion__panel" id="acc-panel-scientific-research" data-accordion="panel" aria-labelledby="acc-button-scientific-research" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/scientists-and-research/58/collaborative-research-in-the-arctic-towards-understanding-climate-change/183">Collaborative Research in the Arctic Towards Understanding Climate Change</a></li> <li><a href="/en/library/scientists-and-research/58/from-stable-chromosomes-to-jumping-genes/184">From Stable Chromosomes to Jumping Genes</a></li> <li><a href="/en/library/scientists-and-research/58/an-elegant-experiment-to-test-the-process-of-dna-replication/187">An Elegant Experiment to Test the Process of DNA Replication</a></li> <li><a href="/en/library/scientists-and-research/58/the-founding-of-neuroscience/233">The Founding of Neuroscience</a></li> <li><a href="/en/library/scientists-and-research/58/tracking-endangered-jaguars-across-the-border/189">Tracking Endangered Jaguars across the Border</a></li> <li><a href="/en/library/scientists-and-research/58/atmospheric-chemistry-research-that-changed-global-policy/211">Atmospheric Chemistry Research that Changed Global Policy</a></li> <li><a href="/en/library/scientists-and-research/58/revolutionizing-medicine-with-monoclonal-antibodies/220">Revolutionizing Medicine with Monoclonal Antibodies</a></li> <li><a href="/en/library/scientists-and-research/58/uncovering-the-mysteries-of-chronic-mountain-sickness/238">Uncovering the Mysteries of Chronic Mountain Sickness</a></li> </ul> </div> <button class="accordion__button" id="acc-button-profiles-in-science" data-accordion="button" aria-controls="acc-panel-profiles-in-science" aria-expanded="false"> <span class="accordion__button__label"> Profiles in Science </span> </button> <div class="accordion__panel" id="acc-panel-profiles-in-science" data-accordion="panel" aria-labelledby="acc-button-profiles-in-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/scientists-and-research/58/luis-e.-miramontes/232">Luis E. Miramontes</a></li> <li><a href="/en/library/scientists-and-research/58/bernardo-houssay/237">Bernardo Houssay</a></li> <li><a href="/en/library/scientists-and-research/58/craig-lee/256">Craig Lee</a></li> <li><a href="/en/library/scientists-and-research/58/david-ho/241">David Ho</a></li> <li><a href="/en/library/scientists-and-research/58/louis-tompkins-wright/244">Louis Tompkins Wright</a></li> <li><a href="/en/library/scientists-and-research/58/carlos-j.-finlay/217">Carlos J. Finlay</a></li> <li><a href="/en/library/scientists-and-research/58/cecilia-payne/290">Cecilia Payne</a></li> <li><a href="/en/library/scientists-and-research/58/jazmin-scarlett/291">Jazmin Scarlett</a></li> <li><a href="/en/library/scientists-and-research/58/ramari-stewart/292">Ramari Stewart</a></li> <li><a href="/en/library/scientists-and-research/58/johnson-cerda/300">Johnson Cerda</a></li> <li><a href="/en/library/scientists-and-research/58/ellen-ochoa/201">Ellen Ochoa</a></li> <li><a href="/en/library/scientists-and-research/58/ruth-benerito/205">Ruth Benerito</a></li> <li><a href="/en/library/scientists-and-research/58/franklin-chang-díaz/219">Franklin Chang Díaz</a></li> <li><a href="/en/library/scientists-and-research/58/percy-lavon-julian/221">Percy Lavon Julian</a></li> <li><a href="/en/library/scientists-and-research/58/luis-walter-alvarez/229">Luis Walter Alvarez</a></li> <li><a href="/en/library/scientists-and-research/58/france-anne-dominic-córdova/230">France Anne-Dominic Córdova</a></li> </ul> </div> </div> </div> </div> </div> </li> <li>  <button class="button" data-toggle="dropdown">Biology </button> <div class="nav__dropdown box-shadow-1 padding-1"> <div class="accordion accordion--secondary font-size-sm"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-sub-button-biological-molecules" data-accordion="button" aria-controls="acc-sub-panel-biological-molecules" aria-expanded="false"> <span class="accordion__button__label"> Biological Molecules </span> </button> <div class="accordion__panel" id="acc-sub-panel-biological-molecules" data-accordion="panel" aria-labelledby="acc-sub-button-biological-molecules" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/carbohydrates/61">Carbohydrates</a></li> <li><a href="/en/library/biology/2/fats-and-proteins/62">Fats and Proteins</a></li> <li><a href="/en/library/biology/2/biological-proteins/243">Biological Proteins</a></li> <li><a href="/en/library/biology/2/blood-biology-i/242">Blood Biology I</a></li> <li><a href="/en/library/biology/2/lipids/207">Lipids</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-cell-biology" data-accordion="button" aria-controls="acc-sub-panel-cell-biology" aria-expanded="false"> <span class="accordion__button__label"> Cell Biology </span> </button> <div class="accordion__panel" id="acc-sub-panel-cell-biology" data-accordion="panel" aria-labelledby="acc-sub-button-cell-biology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64">Discovery and Structure of Cells</a></li> <li><a href="/en/library/biology/2/respiration/285">Respiration</a></li> <li><a href="/en/library/biology/2/membranes-i/198">Membranes I</a></li> <li><a href="/en/library/biology/2/membranes-ii/204">Membranes II</a></li> <li><a href="/en/library/biology/2/cellular-organelles-i/195">Cellular Organelles I</a></li> <li><a href="/en/library/biology/2/cell-division-i/196">Cell Division I</a></li> <li><a href="/en/library/biology/2/cell-division-ii/212">Cell Division II</a></li> <li><a href="/en/library/biology/2/membranes-and-chemical-transport/106">Membranes and Chemical Transport</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-energy-in-living-systems" data-accordion="button" aria-controls="acc-sub-panel-energy-in-living-systems" aria-expanded="false"> <span class="accordion__button__label"> Energy in Living Systems </span> </button> <div class="accordion__panel" id="acc-sub-panel-energy-in-living-systems" data-accordion="panel" aria-labelledby="acc-sub-button-energy-in-living-systems" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/energy-metabolism-i/215">Energy Metabolism I</a></li> <li><a href="/en/library/biology/2/energy-metabolism-ii/225">Energy Metabolism II</a></li> <li><a href="/en/library/biology/2/photosynthesis-i/192">Photosynthesis I</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-evolutionary-biology" data-accordion="button" aria-controls="acc-sub-panel-evolutionary-biology" aria-expanded="false"> <span class="accordion__button__label"> Evolutionary Biology </span> </button> <div class="accordion__panel" id="acc-sub-panel-evolutionary-biology" data-accordion="panel" aria-labelledby="acc-sub-button-evolutionary-biology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/origins-of-life-i/226">Origins of Life I</a></li> <li><a href="/en/library/biology/2/origins-of-life-ii/227">Origins of Life II</a></li> <li><a href="/en/library/biology/2/extinction/295">Extinction</a></li> <li><a href="/en/library/biology/2/mass-extinctions/294">Mass Extinctions</a></li> <li><a href="/en/library/biology/2/charles-darwin-i/110">Charles Darwin I</a></li> <li><a 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aria-labelledby="acc-sub-button-ecology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/biodiversity-i/276">Biodiversity I</a></li> <li><a href="/en/library/biology/2/ecosystem-services/279">Ecosystem Services</a></li> <li><a href="/en/library/biology/2/animal-ecology/283">Animal Ecology</a></li> <li><a href="/en/library/biology/2/biodiversity-ii/281">Biodiversity II</a></li> <li><a href="/en/library/biology/2/animal-behavior/286">Animal Behavior</a></li> <li><a href="/en/library/biology/2/population-biology/287">Population Biology</a></li> <li><a href="/en/library/biology/2/trophic-ecology/293">Trophic Ecology</a></li> </ul> </div> </div> </div> </div> </li> </ul> </nav>  <div id="theTop"></div> <main id="skip-header-content"> <div class="margin-bottom-5"> <article class="container wide module"> <header class="grid grid--sidebar-right module__header"> <div class="module__header__title"> <span class="subcategory"> <strong><em>Genetics</em></strong> </span> <h1>Gene Expression: <sub><em>Enzymes and hereditary conditions</em></sub></h1> <p class="byline">by David Warmflash, MD, Nathan H Lents, Ph.D., Bonnie Denmark, M.A./M.S.</p> <nav class="module__header__tabs"> <ul class="tabs-nav tabs-nav--horizontal library"> <li> <a href="/en/library/biology/2/gene-expression/214/reading" aria-current="page" >Reading</a> </li> <li> <a href="/en/library/biology/2/gene-expression/214/quiz">Quiz</a> </li> <li> <a href="/en/library/biology/2/gene-expression/214/resources">Teach with this</a> </li> </ul> </nav> </div> <script type="application/ld+json"> { "@context": "http://schema.org", "@type": "AudioObject", "contentUrl": "https://www.visionlearning.com/img/library/moduleAudio/module_214.mp3", "description": "Recording of Gene Expression : Through a look at the devastating Tay-Sachs disease and other hereditary conditions, this module explores the connection between genes and enzymes. The role of dominance vs. recessivity is examined. The module traces developments in our understanding of gene expression, starting with a rediscovery of Mendel’s laws of inheritance and built upon by the pioneering work of later scientists. The module introduces the Central Dogma of molecular biology, which is the one-way process of using DNA to make RNA and RNA to make proteins.", "encodingFormat": "mp3", "name": "module_214.mp3" } </script> <div class="module_header_audio"> <div class="audio-player border border-radius"> <audio id="audio"> <source src="https://www.visionlearning.com/img/library/moduleAudio/module_214.mp3" type="audio/mpeg"> Your browser does not support the audio element. </audio> <div class="audio-player__title"> <p>Listen to this reading</p> <span class="audio-player__timestamp" id="timestamp"> 00:00 </span> </div> <div class="audio-player__controls" id="controls"> <button class="button button--icon-only" id="play-pause-button"> <span class="icon icon-play" aria-hidden="true"></span> </button> <div class="audio-player__progress" id="progress-bar" tabindex="0" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-label="Use arrow keys to forward or rewind the audio" role="slider"> <div class="audio-player__progress__fill"> <span class="audio-player__thumb"></span> </div> </div> <div class="audio-player__volume-container"> <button id="mute-button"> <span class="icon icon-volume"></span> </button> <div class="audio-player__volume" tabindex="0" aria-valuemin="0" aria-valuemax="100" aria-valuenow="100" aria-label="Use arrow keys to adjust volume" role="slider"> <div class="audio-player__volume__fill"> <span class="audio-player__thumb"></span> </div> </div> </div> </div> </div> </div> </header> <hr class="divider"/>   <div class="grid grid--sidebar-right grid--divider"> <div class="order-2 order-1--lg module__main"> <div class="narrow margin-x-auto margin-y-5"> <div class="accordion margin-bottom-5">  <button class="accordion__button" id="acc-button-key-concepts" data-accordion="button" aria-controls="acc-panel-key-concepts" aria-expanded="true" tabindex="0"> Did you know? </button> <div class="accordion__panel shown show" id="acc-panel-key-concepts" data-accordion="panel" aria-labelledby="acc-button-key-concepts" role="region"> <div class="accordion__panel__content"> <p>Did you know that blue eyes are the result of defective genes for pigment? Some recessive traits, like eye color, are harmless, while others are deadly. The way that genes translate into physical traits has to do with the particular enzyme that each type of gene makes, a discovery that was made by two scientists by way of the mutant bread mold they created, winning them the Nobel Prize in 1958.</p> </div> </div>  <button class="accordion__button" id="acc-button-table-of-contents" data-accordion="button" aria-controls="acc-panel-table-of-contents" aria-expanded="false" tabindex="0"> Key concepts </button> <div class="accordion__panel" id="acc-panel-table-of-contents" data-accordion="panel" aria-labelledby="acc-button-table-of-contents" role="region" aria-hidden="true"> <div class="accordion__panel__content"> <ul class="bulleted"> <li><p>Genes cannot be used directly by organisms. The information stored in genes must be used to make products, such as enzymes, that cells need to perform different functions. <em>Gene expression</em> is the chemical pathway from genes to the gene products, such as proteins, that organisms can use.</p></li> <li><p>Since organisms have two genes for everything, even If one gene of a pair produces a defective enzyme or no enzyme at all, the other gene in the pair will make enough enzyme to do its job. Only an individual with two genes for a defective enzyme will actually show the recessive trait, such as an inherited disease or condition, blue eyes, or a recessive peapod shape.</p></li> <li><p>In the mid-1900s, George Beadle and Edward Tatum showed that a defective gene leads to a defective enzyme. Their “one gene, one enzyme” hypothesis was later expanded to “one gene, one RNA." </p></li> <li><p>The <em>genetic code</em> is the set of rules that combines amino acids to form polypeptides and is nearly the same for all life-forms on Earth.</p></li> <li><p>The genetic code is not a way for cells to translate genetic information in DNA directly into chains of amino acids to make proteins. Rather, RNA molecules must be made as intermediaries along the way from DNA to the polypeptides that fold into proteins.</p></li> <li><p>Genetic information moves in one direction, from DNA to RNA to protein. This is known as the Central Dogma of molecular biology.</p></li> </ul> </div> </div>  <button class="accordion__button" id="acc-button-terms-you-should-know" data-accordion="button" aria-controls="acc-panel-terms-you-should-know" aria-expanded="false" tabindex="0"> Terms you should know </button> <div class="accordion__panel" id="acc-panel-terms-you-should-know" data-accordion="panel" aria-labelledby="acc-button-terms-you-should-know" role="region" aria-hidden="true"> <div class="accordion__panel__content"> <dl> <dt><a href="/en/glossary/view/enzyme">enzyme </a></dt> <dd> protein-based molecules produced by living cells that bring about or speed up specific biochemical reactions in the organism, such as digestion. </dd> <dt><a href="/en/glossary/view/inherited">inherited </a></dt> <dd> passed down from one generation to another generation </dd> <dt><a href="/en/glossary/view/metabolism">metabolism </a></dt> <dd> the combination of biochemical processes occurring within a living organism to build up or break down materials such as converting food into energy; the sequence of chemical reactions that are necessary to maintain life. </dd> <dt><a href="/en/glossary/view/mutant">mutant </a></dt> <dd> An organism that has undergone a change in genetic material.</dd> </dl> </div> </div> </div> <hr class="border-color-dark" /> <section> <div class="container narrow"> <p>In science, people often have great insights, but they lead to important advances only if science has already laid the foundation for them to be tested. Just as Leonardo da Vinci designed a helicopter-like machine more than 400 years before there would be engines that could make it fly, so was the work of early geneticists like <mark class="term" data-term="Gregor Mendel" data-term-def="Austrian Augustinian monk and scientist born in Heizendorf (now Hynice, Chech Republic) (1822-1884). In 1865, Mendel wrote <i>Versuche über Pflanzen-Hybride (Treatises&hellip;" data-term-url="/en/glossary/view/Mendel%2C+Gregor/4509">Gregor Mendel</mark> and Archibald Garrod too revolutionary to be accepted when it was first shared in the scientific community. Mendel’s ideas on the <mark class="term" data-term="law" data-term-def="In science, a principle that describes a phenomenon, often mathematically." data-term-url="/en/glossary/view/law/8686">laws</mark> of <mark class="term" data-term="inheritance" data-term-def="The transmission of genetic traits from parent to offspring." data-term-url="/en/glossary/view/inheritance/8294">inheritance</mark> were not recognized as truly groundbreaking until after his death. Likewise, when Archibald Garrod posited that certain diseases were <mark class="term" data-term="inherited" data-term-def="Passed down from one generation to another generation." data-term-url="/en/glossary/view/inherited/8738">inherited</mark> from <mark class="term" data-term="parent" data-term-def="The material or source from which something is derived." data-term-url="/en/glossary/view/parent/1618">parents</mark>, science had no way to understand or <mark class="term" data-term="Test" data-term-def="This is a test glossary term." data-term-url="/en/glossary/view/Test/12984">test</mark> his <mark class="term" data-term="hypothesis" data-term-def="From the Greek word <em>hypothesis</em> meaning assumption or the basis of an argument, a hypothesis is a proposal intended to explain&hellip;" data-term-url="/en/glossary/view/hypothesis/3727">hypothesis</mark>.</p> <p>In Garrod's time, the genetic work of <mark class="term" data-term="Gregor Mendel" data-term-def="Austrian Augustinian monk and scientist born in Heizendorf (now Hynice, Chech Republic) (1822-1884). In 1865, Mendel wrote <i>Versuche über Pflanzen-Hybride (Treatises&hellip;" data-term-url="/en/glossary/view/Mendel%2C+Gregor/4509">Gregor Mendel</mark> had only recently been rediscovered (see our <a href="/library/module_viewer.php?mid=129">Mendel and Inheritance</a> module for more information). Through painstaking <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark>, Mendel had shown that <mark class="term" data-term="trait" data-term-def="A specific characteristic that is genetically determined." data-term-url="/en/glossary/view/trait/8271">traits</mark> were passed down from parent to offspring (Figure 1), with some traits being <mark class="term" data-term="dominant" data-term-def="Designating a genetic trait that is expressed when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/dominant/8736">dominant</mark> (showing up in the offspring, even if only one parent carried them) and others being <mark class="term" data-term="recessive" data-term-def="Designating a genetic trait that is hidden when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/recessive/8737">recessive</mark> (can be hidden and skip generations), but nobody knew why this happened. How could someone inherit blue eyes when both parents had brown eyes? Even Mendel was clueless and proposed an almost spiritual mechanism.</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox=""> <img src="/img/library/modules/mid214/Image/VLObject-3221-050208030256.jpg" alt="Figure 1: A Punnet square showing the F1 cross of two plants with alleles Tt. As Mendel observed, 3/4ths of the offspring possess at least one copy of the dominant tall gene T, while 1/4th of the offspring possess two copies of the short gene t." /> </button> <figcaption> <p><strong>Figure 1</strong>: A Punnet square showing the F1 cross of two plants with alleles <em>Tt</em>. As Mendel observed, 3/4ths of the offspring possess at least one copy of the dominant tall gene <em>T</em>, while 1/4th of the offspring possess two copies of the short gene <em>t</em>.</p> </figcaption> </figure> </div> <p>It wasn’t until 1941 that George Beadle and Edward Tatum figured out the mechanism by which <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> are translated into physical <mark class="term" data-term="trait" data-term-def="A specific characteristic that is genetically determined." data-term-url="/en/glossary/view/trait/8271">traits</mark>. The <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark>, known as “gene expression,” is the chemical pathway leading to the particular <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark> that each type of gene makes, resulting in physical characteristics. Beadle and Tatum won the <mark class="term" data-term="Nobel Prize" data-term-def="Awards made annually, beginning in 1901, from funds originally established by Alfred B. Nobel for outstanding achievement in physics, chemistry, medicine&hellip;" data-term-url="/en/glossary/view/Nobel+Prize/3843">Nobel Prize</mark> for their work in 1958, nearly a century after Mendel published his <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> on the <mark class="term" data-term="inheritance" data-term-def="The transmission of genetic traits from parent to offspring." data-term-url="/en/glossary/view/inheritance/8294">inheritance</mark> of genetic traits. </p> <p><section id="toc_1" class=""> <h2>Shedding light on a hereditary disease: Waren Tay and Bernard Sachs</h2></p> <p>Although <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> were still completely <mark class="term" data-term="abstract" data-term-def="In science, an abstract is a brief statement of essential information contained within a document or presentation. An abstract is not&hellip;" data-term-url="/en/glossary/view/abstract/5214">abstract</mark> in the late 18th and early 19th centuries, researchers were starting to recognize that certain diseases ran in families. One particularly devastating condition manifested itself with a range of symptoms in the central nervous <mark class="term" data-term="system" data-term-def="A group of interacting, interrelated or interdependent components that form a complex whole. The size of the system is defined for&hellip;" data-term-url="/en/glossary/view/system/3904">system</mark>. Afflicted infants looked normal at birth, but gradually developed mental and physical retardation, leading to paralysis, blindness, deafness, and ultimately death, usually by age three. The disease has been around for ages, but only in the 19<sup>th</sup> century had medicine advanced enough to recognize it. Various technological advances by 19<sup>th</sup> century lens grinders allowed for major improvements in telescopes and microscopes, leading to some well-known discoveries in astronomy and biology. Alongside those improving telescopes and microscopes came a new invention: the ophthalmoscope. That’s the instrument that doctors use to examine the retinas of your eyes. It was invented in 1851, and by the 1880s was already the most important tool for ophthalmologists. Using one to examine a child with mental and physical retardation whose vision was also deteriorating, Waren Tay, a British ophthalmologist in London, noticed something in the retina that was not supposed to be there. He called it a “cherry red spot” (Figure 2), and in his report for a medical journal he noted that the child was Jewish. </p><p>An ocean away from Tay’s London practice, a New York pediatric neurologist, Bernard Sachs, was being sent all of the unusual neurologic cases in the city. Many of the patients were part of a new wave of immigrants to the city that included massive numbers of Jews from Central and Eastern Europe. After seeing a few cases of deteriorating physical and mental retardation, Sachs began looking at the brains of children who had died. Observing the same kind of swelling in the nerve <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> from autopsy samples, Sachs came to realize that the patients were afflicted by the same disease. By questioning the parents of the children to see if they recalled stories of similar cases in their villages back in the old country, he also figured out that the condition ran in families of Jews. Calling the condition infantile amaurotic familial idiocy, Sachs noted that it skipped <mark class="term" data-term="generation" data-term-def="Offspring at the same step in the line of descent from a common ancestor." data-term-url="/en/glossary/view/generation/8293">generations</mark>, usually more than one generation at a time, before showing up in another infant, such as the child or grandchild.</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox=""> <img src="/img/library/modules/mid214/Image/VLObject-8616-150731020757.jpg" alt="Figure 2: The "cherry red spot" as observed by Tay in his work as an ophthalmologist." /> </button> <figcaption> <p><strong>Figure 2</strong>: The "cherry red spot" as observed by Tay in his work as an ophthalmologist.</p> <span class="credit">image ©Jonathan Trobe, M.D., U. Michigan</span> </figcaption> </figure> </div> </section> <section id="toc2_1"><h3>Gene expression</h3><p>Eventually, Tay and Sachs (Figure 3) realized that they were studying the same condition. Today, it’s called <em>Tay-Sachs disease</em>, and the cherry red spot that Tay saw in his ophthalmoscope is a telltale sign. Reporting that the disease skipped <mark class="term" data-term="generation" data-term-def="Offspring at the same step in the line of descent from a common ancestor." data-term-url="/en/glossary/view/generation/8293">generations</mark>, Sachs actually was implying that it displayed what Mendel termed a “recessive factor.” Just like the <mark class="term" data-term="recessive" data-term-def="Designating a genetic trait that is hidden when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/recessive/8737">recessive</mark> shapes and colors of Mendel’s peapods, and just like blue eye color or straight hair, Tay-Sachs disease is caused by a specific version of a <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark>, but only if two copies of the version are present. While Sachs did not express his <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">observations</mark> in Mendel’s terminology, this was around the time when Mendel’s <mark class="term" data-term="law" data-term-def="In science, a principle that describes a phenomenon, often mathematically." data-term-url="/en/glossary/view/law/8686">laws</mark> were being rediscovered. Along with new instruments and <mark class="term" data-term="method" data-term-def="A procedure or process; a systematic way of performing a task or conducting research." data-term-url="/en/glossary/view/method/8238">methods</mark> shaping early 20th century science, those rediscovered laws beckoned to a new generation of geneticists.</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox=""> <img src="/img/library/modules/mid214/Image/VLObject-8615-150731020746.jpg" alt="Figure 3: The two scientists behind the discovery of Tay-Sachs disease: Bernard Sachs (l), a New York pediatric neurologist, and Waren Tay (r), a British ophthalmologist." /> </button> <figcaption> <p><strong>Figure 3</strong>: The two scientists behind the discovery of Tay-Sachs disease: Bernard Sachs (l), a New York pediatric neurologist, and Waren Tay (r), a British ophthalmologist.</p> </figcaption> </figure> </div> <p>But Mendel’s <mark class="term" data-term="law" data-term-def="In science, a principle that describes a phenomenon, often mathematically." data-term-url="/en/glossary/view/law/8686">laws</mark> do not explain how dominance and recessivity work. How could it be that an infant gets a terrible disease, dies in childhood, and therefore does not grow up to have children, yet later the same disease reappears in a nephew or niece, or in a grandchild of the infant’s sibling? What is the path from a particular <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> to the manifestation of a certain disease, condition, or trait? This <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> would not be understood until later researchers continued investigating the mechanism of gene expression over another several decades.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form class="" name="cc7831"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Tay-Sachs disease</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7831-0-option-a" name="quiz-option-7831" type="radio" value="skips generations in families. " > <span class="option__label"> <span class="screen-reader-only">a.</span> skips generations in families. </span> </label> <span class="quiz__response" id="response-7831-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7831-1-option-b" name="quiz-option-7831" type="radio" value="appears in every generation in families." > <span class="option__label"> <span class="screen-reader-only">b.</span> appears in every generation in families. </span> </label> <span class="quiz__response" id="response-7831-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_2"> <h2>Enzymes and hereditary conditions: Archibald Garrod</h2><p>Not far from Tay’s ophthalmology clinic in London, another medical doctor was conducting <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> that would lead to a major breakthrough in our understanding of <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> expression. Archibald Garrod was studying people with a handful of medical conditions. Much more benign than Tay-Sachs disease, the conditions that piqued Garrod’s interest did not kill the patients as toddlers. At the same time, each condition came with a telltale <mark class="term" data-term="trait" data-term-def="A specific characteristic that is genetically determined." data-term-url="/en/glossary/view/trait/8271">trait</mark>. One condition that Garrod studied, called <em>albinism</em>, leaves children with no <mark class="term" data-term="pigment" data-term-def="A light-absorbing molecule that gathers energy from the sun; a molecule that gives color and is involved in vital functions within&hellip;" data-term-url="/en/glossary/view/pigment/8522">pigment</mark> in their hair, eyes, or skin. He was also fascinated by <em>cystinuria</em>, a condition characterized by frequent urinary stones beginning in early adulthood, and another condition that produces urine that darkens when left standing. Known as <em>alkaptonuria</em>, it typically is discovered after a parent notices dark stains in an infant’s diapers. Realizing that urine provided an easy way to study the chemistry of the body, Garrod also took urine samples from people whose health seemed perfectly normal. In doing so, he discovered another condition, called <em>pentosuria</em>, whose only sign is the presence of a certain kind of <mark class="term" data-term="sugar" data-term-def="A water-soluble crystalline carbohydrate. There are many types of sugar of varying degrees of sweetness, including fructose, which occurs naturally in&hellip;" data-term-url="/en/glossary/view/sugar/5309">sugar</mark> in the urine.</p><p>Some of Garrod’s conditions also produce other effects that were not so easily recognized in those days. People with alkaptonuria, for instance, often develop trouble in large joints, disks of the spine, and heart valves as they age. But most of these problems appear long after the patients can grow up and have children of their own, and, in the case of pentosuria, there are no known detrimental effects on health. These features made investigating family connections much easier for Garrod than for Sachs.</p><p>Knowledgeable of the newly rediscovered Mendelian <mark class="term" data-term="law" data-term-def="In science, a principle that describes a phenomenon, often mathematically." data-term-url="/en/glossary/view/law/8686">laws</mark>, Garrod hypothesized that a single <mark class="term" data-term="recessive" data-term-def="Designating a genetic trait that is hidden when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/recessive/8737">recessive</mark> <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> was the cause of each condition, and the gene was passed down in particular family lines (Figure 4). For instance, though pentosuria was the most benign disease of all the four that Garrod studied, it had something in common with the deadly Tay-Sachs disease; namely, it ran in Jewish families. Going beyond Mendel and Sachs, however, Garrod also suggested that for each condition, a recessive gene caused a deficiency of an <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark> whose normal role was to create, break down, or modify a particular chemical.</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox="" data-lightbox-src="/img/library/large_images/image_8618.jpg"> <img src="/img/library/modules/mid214/Image/VLObject-8618-150731030731.jpg" alt="Figure 4: Garrod theorized that the diseases he studied, including Tay-Sachs disease, were inherited from the parents. He correctly believed the diseases were caused by a recessive gene in the children was causing an enzyme deficiency." /> </button> <figcaption> <p><strong>Figure 4</strong>: Garrod theorized that the diseases he studied, including Tay-Sachs disease, were inherited from the parents. He correctly believed the diseases were caused by a recessive gene in the children was causing an enzyme deficiency.</p> <span class="credit">image ©Cburnett</span> </figcaption> </figure> </div> </section> <section id="toc2_2"><h3>The role of enzymes and inborn errors of metabolism</h3><p>It was an amazing stroke of insight, for Garrod was correct. Manufactured in all plant and animal <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark>, <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> are <mark class="term" data-term="catalyst" data-term-def="A substance that speeds up the rate of a chemical reaction but that is not used up in the process." data-term-url="/en/glossary/view/catalyst/8234">catalysts</mark> that enable biochemical <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> to move forward at a faster rate, speeding up reactions that would take much longer. Enzymes are vital to many of the body’s critical <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">functions</mark>; without them, <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark> would not be able to survive and function. These predominantly protein-based <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecules</mark> perform very specific tasks within the body (Figure 5). Understanding their role was key to a new understanding <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> expression.</p><p>Each of the four conditions that Garrod studied really does result from a problem with a single <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark>. The same enzyme problem that causes dark urine in alkaptonuria also affects cartilage and other connective tissues throughout the body, thereby affecting the joints, spinal discs, and heart valves. This happens because the one enzyme that’s affected in alkaptonuria happens to control the breakdown of two of the 20 amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark> that life-forms use for just about everything. Garrod didn’t work out this <mark class="term" data-term="amino acid" data-term-def="Biochemical molecules that contain at least one amine group (-NH<sub>2</sub>) and at least one carboxylic acid group (-COOH) and conform&hellip;" data-term-url="/en/glossary/view/amino+acid/1596">amino acid</mark> chemistry, but studying the families of patients with alkaptonuria and the other abnormalities, he developed a concept called <em>inborn errors of metabolism</em>. The <mark class="term" data-term="hypothesis" data-term-def="From the Greek word <em>hypothesis</em> meaning assumption or the basis of an argument, a hypothesis is a proposal intended to explain&hellip;" data-term-url="/en/glossary/view/hypothesis/3727">hypothesis</mark> was way ahead of its time, yet Garrod had no way to <mark class="term" data-term="Test" data-term-def="This is a test glossary term." data-term-url="/en/glossary/view/Test/12984">test</mark> it, so it did not catch on during his lifetime. Unlike da Vinci’s helicopter, though, Garrod’s vindication lay not four centuries into the future, but a mere four decades.</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox="" data-lightbox-src="/img/library/large_images/image_8619.jpg"> <img src="/img/library/modules/mid214/Image/VLObject-8619-150803110801.jpg" alt="Figure 5: This diagram shows how enzymes enable biochemical reactions to move forward by catalyzing a single reaction." /> </button> <figcaption> <p><strong>Figure 5</strong>: This diagram shows how enzymes enable biochemical reactions to move forward by catalyzing a single reaction.</p> </figcaption> </figure> </div> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form class="" name="cc7837"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Garrod proposed that</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7837-0-option-a" name="quiz-option-7837" type="radio" value="amino acids were the building blocks of protein." > <span class="option__label"> <span class="screen-reader-only">a.</span> amino acids were the building blocks of protein. </span> </label> <span class="quiz__response" id="response-7837-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7837-1-option-b" name="quiz-option-7837" type="radio" value="the lack of an enzyme led to certain health conditions." > <span class="option__label"> <span class="screen-reader-only">b.</span> the lack of an enzyme led to certain health conditions. </span> </label> <span class="quiz__response" id="response-7837-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_3"> <h2>Making mutants: George Beadle and Edward Tatum</h2><p>During the late 1930s, the final years of Garrod’s life, George Beadle was a young geneticist at Columbia University, where he was doing <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> on fruit flies called <em>Drosophila melanogaster</em>. Fruit flies, like humans, have noticeable differences in eye color that follow Mendelian <mark class="term" data-term="inheritance" data-term-def="The transmission of genetic traits from parent to offspring." data-term-url="/en/glossary/view/inheritance/8294">inheritance</mark>. Using <mark class="term" data-term="radiation" data-term-def="Energy emitted as particles, waves, or rays." data-term-url="/en/glossary/view/radiation/8266">radiation</mark> to damage the <em>Drosophila</em> <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> – whatever they were, for nobody yet knew their physical basis – Beadle was able to show that genes were related to eye color through a series of <mark class="term" data-term="chemical reaction" data-term-def="A process in which atoms and molecules recombine by forming or breaking chemical bonds. Chemical reactions form new products that&hellip;" data-term-url="/en/glossary/view/chemical+reaction/1547">chemical reactions</mark>. Still, he couldn’t be sure whether the idea could apply to a wide range of <mark class="term" data-term="trait" data-term-def="A specific characteristic that is genetically determined." data-term-url="/en/glossary/view/trait/8271">traits</mark> and to life in general, or merely to eye color in fruit flies.</p><p>Teaming up with biochemist Edward Tatum in 1940, Beadle set aside the fruit flies in favor of <em>Neurospora crassa</em>, a type of bread mold (Figure 6). Like peas and people, fruit flies have two sets of <mark class="term" data-term="chromosome" data-term-def="The organized genetic structure of DNA with associated proteins that contains the hereditary information necessary for reproduction, protein manufacture, and other functions." data-term-url="/en/glossary/view/chromosome/3760">chromosomes</mark> that carry <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> for different characteristics. Thus, two genes encode the information for each <mark class="term" data-term="trait" data-term-def="A specific characteristic that is genetically determined." data-term-url="/en/glossary/view/trait/8271">trait</mark>, which is the cause of dominance and recessivity. Unlike fruit flies, <em>N. crassa</em> can produce little reproductive structures called spores that carry just one set of chromosomes, so dominance and recessivity do not come into play. Also, <em>N. crassa</em> offered another advantage. Studying fruit fly genetics, Beadle had to look at physical effects, like eye color, and picking up flies with tweezers can be time-consuming. <em>N. crassa</em> spores, on the other hand, could be placed on top of a nutrient-filled gel that has solidified, and Beadle could simply observe whether or not spores grew on the gel. </p><p>Since the answer was either “growing” or “not growing,” he could have hundreds of gel-filled plates, each with a spore. The <mark class="term" data-term="nutrient" data-term-def="A chemical substance (e.g., minerals, vitamins, proteins) that is needed by an organism to survive and grow. See also: macronutrient and micronutrient." data-term-url="/en/glossary/view/nutrient/7058">nutrient</mark> gel contained only the minimal number of nutrients, the essential <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compounds</mark>, which the spores normally needed to grow (sugar, certain <mark class="term" data-term="salt" data-term-def="Generally, any ionic compound except those that contain hydroxide or hydrogen ions. Specifically, any compound other than water formed by&hellip;" data-term-url="/en/glossary/view/salt/1575">salts</mark>, and a vitamin called biotin). All other important chemical compounds the spores could make themselves, using the essential nutrients supplied in the gel as starting compounds. By using different nutrient <mark class="term" data-term="mixtures" data-term-def="two or more pure substances combined that retain their unique properties. Mixtures include homogeneous mixtures, also called or solutions, where one&hellip;" data-term-url="/en/glossary/view/mixtures/12622">mixtures</mark>, Beadle could observe whether a particular spore needed an extra nutrient, an ingredient not usually included in the gel since normal spores can make it themselves. Any spores needing an extra nutrient in order to grow could be considered abnormal. In genetics, these spores are called <em>mutants</em>, while the others (those able to grow with no extra ingredients) are called the <em>wild type</em>. If Garrod was right and each <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> produced a certain <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark>, then damaging the gene for the enzyme that an <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organism</mark> used to make nutrient X would create a <em>mutant</em> organism that could grow only if nutrient X was supplied from the outside.</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox=""> <img src="/img/library/modules/mid214/Image/VLObject-8620-150803110842.jpg" alt="Figure 6: Neurospora crassa, a type of red bread mold studied by Beadle and Tatum." /> </button> <figcaption> <p><strong>Figure 6</strong>: <em>Neurospora crassa</em>, a type of red bread mold studied by Beadle and Tatum.</p> <span class="credit">image ©Jamie Cate</span> </figcaption> </figure> </div> <p>Although Beadle and Tatum did not know the physical basis of the <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark>, they were certain that each <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organism</mark> carried a whole lot of genes, probably thousands. In that case, how could they hope to create a <mark class="term" data-term="mutant" data-term-def="An organism that has undergone a change in genetic material." data-term-url="/en/glossary/view/mutant/8739">mutant</mark> that depended on one and only one particular <mark class="term" data-term="nutrient" data-term-def="A chemical substance (e.g., minerals, vitamins, proteins) that is needed by an organism to survive and grow. See also: macronutrient and micronutrient." data-term-url="/en/glossary/view/nutrient/7058">nutrient</mark> simply by zapping the organism with radiation? Actually, they weren’t sure that they could, but they understood the power in numbers. Like hoping to draw the queen of hearts from a shuffled deck of cards, you might get lucky if you try enough times. Likewise, when you can spread mold spores on a series of culture plates, you have more chances than when you’re picking up flies with tweezers. Even so, the scientists thought it could be a long shot, and so they made a deal. They would irradiate sample after sample and check mutant after mutant to see how they could grow or not grow with the addition or absence of particular nutrients. But they would set a limit of 5,000 attempts. If they got to that point without creating the mutant they needed, they would give up.</p><p>But they never had to give up because after just a few hundred attempts, they found a <mark class="term" data-term="mutant" data-term-def="An organism that has undergone a change in genetic material." data-term-url="/en/glossary/view/mutant/8739">mutant</mark> that needed just one ingredient added to the usual growth mixture. That needed extra <mark class="term" data-term="nutrient" data-term-def="A chemical substance (e.g., minerals, vitamins, proteins) that is needed by an organism to survive and grow. See also: macronutrient and micronutrient." data-term-url="/en/glossary/view/nutrient/7058">nutrient</mark> was <em>arginine</em>, one of the 20 amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark> that life-forms use as building blocks to make <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark>. Normal <em>N. crassa</em> can make its own arginine, but Beadle and Tatum were able to create four different molds that could only survive when given arginine in their food. Using these <mark class="term" data-term="strain" data-term-def="A group of closely related organisms; a distinct variety, as in a strain of bacteria." data-term-url="/en/glossary/view/strain/8288">strains</mark>, they were able to trace the chemical pathways connected with the mutated <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> of the strains, ultimately demonstrating that each <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark> was made by one particular gene. Published in 1941, it was a milestone discovery that eventually would earn Beadle and Tatum the <mark class="term" data-term="Nobel Prize" data-term-def="Awards made annually, beginning in 1901, from funds originally established by Alfred B. Nobel for outstanding achievement in physics, chemistry, medicine&hellip;" data-term-url="/en/glossary/view/Nobel+Prize/3843">Nobel Prize</mark>. Their discovery was not limited to bread molds, for gradually it became clear that Tay-Sachs, all four of the conditions that Garrod studied, and a host of other familial disease were due to <mark class="term" data-term="recessive" data-term-def="Designating a genetic trait that is hidden when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/recessive/8737">recessive</mark> gene mutants.</p><div class='"table-container"'><table class="table" aria-describedby="configDescription"> <caption id="configDescription"> <strong>Table 1:</strong> <em>Neurospora crassa</em> Experiment Growth Data. Normal <em>N. crassa</em> (aka, the "wild type") can make its own arginine, but Beadle and Tatum were able to create four different molds (ARG-E, ARG-F, ARG-G, and ARG-H) that could only survive when given arginine in their food. </caption> <thead> <tr> <th scope="col">Mutant strain</th> <th scope="col">No supplement</th> <th scope="col">Ornithine</th> <th scope="col">Citrulline</th> <th scope="col">Arginino-succinate</th> <th scope="col">Arginine</th> </tr> </thead> <tbody> <tr> <td scope="row"><em>Wild type</em></td> <td>+</td> <td>+</td> <td>+</td> <td>+</td> <td>+</td> </tr> <tr> <td scope="row"><em>ARG-E</em></td> <td>-</td> <td>+</td> <td>+</td> <td>+</td> <td>+</td> </tr> <tr> <td scope="row"><em>ARG-F</em></td> <td>-</td> <td>-</td> <td>+</td> <td>+</td> <td>+</td> </tr> <tr> <td scope="row"><em>ARG-G</em></td> <td>-</td> <td>-</td> <td>-</td> <td>+</td> <td>+</td> </tr> <tr> <td scope="row"><em>ARG-H</em></td> <td>-</td> <td>-</td> <td>-</td> <td>-</td> <td>+</td> </tr> </tbody> </table></div> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form class="" name="cc7843"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Beadle and Tatum worked with the <em>Neurospora crassa</em> bread mold instead of fruit flies because</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7843-0-option-a" name="quiz-option-7843" type="radio" value="the mold exhibited many different physical effects that could be observed." > <span class="option__label"> <span class="screen-reader-only">a.</span> the mold exhibited many different physical effects that could be observed. </span> </label> <span class="quiz__response" id="response-7843-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7843-1-option-b" name="quiz-option-7843" type="radio" value="the mold was easy to observe." > <span class="option__label"> <span class="screen-reader-only">b.</span> the mold was easy to observe. </span> </label> <span class="quiz__response" id="response-7843-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc2_3"><h3>Gene mutants and health conditions </h3><p>How does it work? Usually, for converting and breaking down chemicals in the body, <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> are in fairly good supply. Like humans and peas, having two <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> for everything, including for enzymes, means that you have a backup. If one gene of a pair is mutated and produces a defective enzyme or no enzyme at all, the individual still has the other gene, which makes enough enzyme to break down the chemical, convert the chemical to something else, or do whatever the enzyme does. Only an individual with two genes for the defective enzyme of alkaptonuria actually has the disease, just as two genes for a defective <mark class="term" data-term="pigment" data-term-def="A light-absorbing molecule that gathers energy from the sun; a molecule that gives color and is involved in vital functions within&hellip;" data-term-url="/en/glossary/view/pigment/8522">pigment</mark> are needed for a person to be an albino. A similar thing happens with human eye color. The gene for brown irises (the colored part of the eye) produces a dark pigment, which, if absent, leaves the iris blue. The dark color shows up, eliminating the blue, even if the individual has only one gene for brown eyes, which is why brown eyes are <mark class="term" data-term="dominant" data-term-def="Designating a genetic trait that is expressed when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/dominant/8736">dominant</mark>. Blue eyes are <mark class="term" data-term="recessive" data-term-def="Designating a genetic trait that is hidden when an organism has inherited two different variations (alleles) of a gene for that&hellip;" data-term-url="/en/glossary/view/recessive/8737">recessive</mark>, because having them means you have no brown pigment at all, which only happens if both of your pigment genes are defective (Figure 7).</p> <div class="figure"> <figure> <button class="lightbox-button lightbox-button--icon" data-lightbox="" data-lightbox-src="/img/library/large_images/image_8622.png"> <img src="/img/library/modules/mid214/Image/VLObject-8622-150806030809.png" alt="Figure 7: A Punnett square showing how eye color develops. Here, a brown-eyed parent and a blue-eyed parent produce 50% children with brown eyes (a dominant trait) and 50% children with blue eyes (a recessive trait)." /> </button> <figcaption> <p><strong>Figure 7</strong>: A Punnett square showing how eye color develops. Here, a brown-eyed parent and a blue-eyed parent produce 50% children with brown eyes (a dominant trait) and 50% children with blue eyes (a recessive trait).</p> <span class="credit">image ©Purpy Pupple</span> </figcaption> </figure> </div> <p>The <mark class="term" data-term="principle" data-term-def="In the sciences, a principle is a fundamental, primary, or general law or truth. For instance, one of the most basic&hellip;" data-term-url="/en/glossary/view/principle/5289">principle</mark> also carries over to Tay-Sachs disease. Today, we know the disease is caused by an inability to break down a category of chemicals called <em>lipids</em>, specifically a special type of lipid called a <em>GM2 ganglioside</em>. Extremely important in <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membranes</mark> of nerve <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> or <em>neurons</em>, GM2 ganglioside is broken down by an <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark> called <em>HEXA</em>. GM2 ganglioside gradually accumulates in the neurons of a child who makes no HEXA, but the accumulation takes time, which is why newborns with Tay-Sachs appear normal. Over months, however, the accumulating GM2 ganglioside causes the neurons to swell. Since the retina of the eye is made of the same kind of neurons that are in the brain, the retina swells in a particular pattern, and that’s what causes the cherry red spot that’s characteristic of Tay-Sachs disease, and eventually blindness. Similar swelling throughout the brain causes all of the other symptoms, and finally death.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form class="" name="cc7847"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">A person who is an albino or who has the disease <em>alkaptonuria</em> must have _________ for the defective enzyme that causes these conditions.</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7847-0-option-a" name="quiz-option-7847" type="radio" value="one gene" > <span class="option__label"> <span class="screen-reader-only">a.</span> one gene </span> </label> <span class="quiz__response" id="response-7847-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7847-1-option-b" name="quiz-option-7847" type="radio" value="two genes" > <span class="option__label"> <span class="screen-reader-only">b.</span> two genes </span> </label> <span class="quiz__response" id="response-7847-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc2_4"><h3>From genes to protein enzymes</h3><p>Beadle and Tatum’s demonstration that a defective <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> leads to a defective <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzyme</mark> proved that enzymes were made as a consequence of genes. Published in 1941, this was a watershed discovery in genetics that set the stage for other researchers to hone in on the physical basis of genes and on gene expression, the chemical pathways from genes to <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">protein</mark> enzymes. The particular arrangement of <mark class="term" data-term="atom" data-term-def="The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in&hellip;" data-term-url="/en/glossary/view/atom/1509">atoms</mark> within a gene allows for storage of information. When that stored genetic information is used to make enzymes, the gene is expressed, and Beadle and Tatum set the stage for new researchers to discover how the gene expression <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> worked. The stories of those other researchers are recounted in the modules that focus on the genes and on each phase of the process leading to the manufacture of their products.</p><p>The <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark> <mark class="term" data-term="product" data-term-def="The material that is formed as a result of a chemical reaction. Written on the right side of a chemical equation.&hellip;" data-term-url="/en/glossary/view/product/1569">products</mark> do not include only <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark>, but enzymes were the first gene products to be understood. All of the enzymes affected in genetic diseases, like those studied by Garrod, Tay, and Sachs, are <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark>. Immensely versatile and complex, proteins come from amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark>, which are linked in a chain called a <em>polypeptide</em>. When properly folded, one or more polypeptides form a protein. In addition to being enzymes, proteins take on a variety of roles, from providing structure for biological tissues to carrying important <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecules</mark> around the body and in and out of <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark>.</p></section> <section id="toc_4"> <h2>The Central Dogma of molecular biology</h2><p>Using the Beadle-Tatum discovery as a starting point, biologists during the 1940s and 50s figured out not just that <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> carried the <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark>, but they started to get an idea of how those genes were replicated and passed from <mark class="term" data-term="generation" data-term-def="Offspring at the same step in the line of descent from a common ancestor." data-term-url="/en/glossary/view/generation/8293">generation</mark> to generation. Soon after that they learned that amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark> were put together into polypeptides using a set of rules called the <em>genetic code</em>, which is nearly the same for all life-forms on Earth. They also learned that the genetic code was not a way for <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> to translate genetic information in DNA directly into chains of <mark class="term" data-term="amino acid" data-term-def="Biochemical molecules that contain at least one amine group (-NH<sub>2</sub>) and at least one carboxylic acid group (-COOH) and conform&hellip;" data-term-url="/en/glossary/view/amino+acid/1596">amino acids</mark> to make <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark>. Instead, there are <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecules</mark> called <em>RNA</em> that must be made as intermediaries along the way from DNA to the polypeptides that fold into proteins. The <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> of using DNA to make <mark class="term" data-term="RNA" data-term-def="Ribonucleic acid. A single-stranded nucleic acid containing the sugar ribose. In most organisms, a molecule responsible for transfer of the&hellip;" data-term-url="/en/glossary/view/RNA/1605">RNA</mark>, and then RNA to make polypeptides is one-directional. Never is a sequence of <mark class="term" data-term="amino acid" data-term-url="/en/glossary/view/amino+acid" data-term-def="Biochemical molecules that contain at least one amine group (-NH<sub>2</sub>) and at least one carboxylic acid group (-COOH) and conform&hellip;">amino acids</mark> of a polypeptide used as a message for making either RNA or DNA, and only in certain viral infections is RNA ever used to make DNA. Known as the <em>Central Dogma of molecular biology</em>, this one-way process is universal to all <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark>.</p><p>The one-directional nature of the movement of genetic information, which scientists came to understand from the 1950s-1970s, rests upon Beadle and Tatum’s 1941 watershed discovery. Before anyone could identify <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> as the physical basis of <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark>, and before anyone could reveal the chemistry carrying the genetic messages from DNA to <mark class="term" data-term="RNA" data-term-def="Ribonucleic acid. A single-stranded nucleic acid containing the sugar ribose. In most organisms, a molecule responsible for transfer of the&hellip;" data-term-url="/en/glossary/view/RNA/1605">RNA</mark>, and finally to the amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark> that make a polypeptide, somebody had to show what chemical <mark class="term" data-term="product" data-term-def="The material that is formed as a result of a chemical reaction. Written on the right side of a chemical equation.&hellip;" data-term-url="/en/glossary/view/product/1569">product</mark> genes were actually affecting. And in the case of the various <mark class="term" data-term="inherited" data-term-def="Passed down from one generation to another generation." data-term-url="/en/glossary/view/inherited/8738">inherited</mark> diseases, it was <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark>. This supported the <mark class="term" data-term="hypothesis" data-term-def="From the Greek word <em>hypothesis</em> meaning assumption or the basis of an argument, a hypothesis is a proposal intended to explain&hellip;" data-term-url="/en/glossary/view/hypothesis/3727">hypothesis</mark> of “one gene, one enzyme,” which was expanded to “one gene, one polypeptide” after it was realized that non-enzyme <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark> were also made using genes, and eventually it had to be expanded again. Although many RNA <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecules</mark> carry actual genetic messages from DNA that are used to make polypeptides, the job of some other RNA molecules is to help with the <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark>. Since the helper RNA molecules also are made from genes, it’s not accurate to say that all genes code for some kind of protein product. Thus, today we say "one gene, one RNA."</p><p>Even before Beadle and Tatum could prove “one <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">gene</mark>, one enzyme” with their painstaking bread mold <mark class="term" data-term="experiment" data-term-def="A test or trial carried out under controlled conditions so that specific actions can be performed and the results can be observed." data-term-url="/en/glossary/view/experiment/8292">experiments</mark>, somebody had to imagine a connection between genes and <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> in the first place. Poor Archibald Garrod had died in 1936, just five years short of the Beadle and Tatum publication that vindicated the idea of inborn errors of <mark class="term" data-term="metabolism" data-term-def="A sequence of biochemical reactions in living organisms that converts food into energy used to drive other biological processes. Also, the&hellip;" data-term-url="/en/glossary/view/metabolism/1606">metabolism</mark>. But Beadle and Tatum did remember Garrod. Inspired by their predecessor, Beadle named him at their 1958 <mark class="term" data-term="Nobel Prize" data-term-def="Awards made annually, beginning in 1901, from funds originally established by Alfred B. Nobel for outstanding achievement in physics, chemistry, medicine&hellip;" data-term-url="/en/glossary/view/Nobel+Prize/3843">Nobel Prize</mark> acceptance as the ultimate inspiration for their work.</p> </div> </section> <hr class="border-color-dark" /> <footer class="module__footer"> <p class="citation"> <em> David Warmflash, MD, Nathan H Lents, Ph.D., Bonnie Denmark, M.A./M.S. “Gene Expression” Visionlearning Vol. BIO-4 (4), 2015. </em> </p>  <div class="title-list" name="further"> <p class="h6 title-list__title"> Further Reading </p> <ul class="grid grid--column-2--md grid--column-3--md gap-1"> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Mendel-and-Inheritance/129"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_129-23061209063555.jpg" alt="Inheritance"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Inheritance: <em>Mendel's experiments and laws</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Mendel-and-Independent-Assortment/145"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_145-23061209063608.jpg" alt="Independent Assortment"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Independent Assortment: <em>Mendel's testcrosses and Punnett squares</em> </h2> </div> </article> </a> </li> </ul> </div> </footer> </div>   </div>  <div class="order-1 order-2--lg module__tools"> <div class="narrow margin-x-auto position-sticky-top font-size-md"> <div class="padding-2 border-radius box-shadow-1--lg"> <div class="tabs" role="tablist"> <nav> <button class="button button--icon-label" id="tab-button-in-this-module" aria-label="Table of Contents" aria-controls="tab-panel-module__tools" aria-selected="true" role="tab"> <span class="icon icon-list" aria-hidden="true"></span> <span class="button__text">Contents</span> </button> <button class="button button--icon-label" id="tab-button-toggle-terms" aria-controls="tab-panel-toggle-terms" aria-selected="false" role="tab"> <span class="icon icon-glossary-highlight"></span> <span class="button__text">Glossary Terms</span> </button> </nav> <hr class="divider" /> <div class="tabs__panel shown" id="tab-panel-module__tools" aria-labelledby="tab-button-module__tools" role="tabpanel"> <p class="font-weight-bold margin-bottom-1"> Table of Contents </p> <div class="table-of-contents" id="module-toc"> <ul> <li><a href="/en/library/biology/2/gene-expression/214#toc_1">Shedding light on a hereditary disease: Waren Tay and Bernard Sachs</a> </li> <li> <ul> <li><a href="/en/library/biology/2/gene-expression/214#toc2_1">Gene expression</a> </li> </ul> </li> <li><a href="/en/library/biology/2/gene-expression/214#toc_2">Enzymes and hereditary conditions: Archibald Garrod</a> </li> <li> <ul> <li><a href="/en/library/biology/2/gene-expression/214#toc2_2">The role of enzymes and inborn errors of metabolism</a> </li> </ul> </li> <li><a href="/en/library/biology/2/gene-expression/214#toc_3">Making mutants: George Beadle and Edward Tatum</a> </li> <li> <ul> <li><a href="/en/library/biology/2/gene-expression/214#toc2_3">Gene mutants and health conditions </a> </li> </ul> </li> <li> <ul> <li><a href="/en/library/biology/2/gene-expression/214#toc2_4">From genes to protein enzymes</a> </li> </ul> </li> <li><a href="/en/library/biology/2/gene-expression/214#toc_4">The Central Dogma of molecular biology</a> </li> </ul> </div> </div>   <div class="tabs__panel" id="tab-panel-toggle-terms" aria-labelledby="tab-button-toggle-terms" role="tabpanel"> <div class="reading-toggle"> <div class="reading-toggle__switch"> <div class="form-entry__option__switch"> <label> <input type="checkbox" name="termsToggleSwitch" id="terms-toggle-switch" /> <span class="switch__slider"></span> <span class="option__label text-decoration-none font-size-md"> Highlight Glossary Terms </span> </label> </div> </div> <div class="reading-toggle__help"> <p> <em> Activate glossary term highlighting to easily identify key terms within the module. 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