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Evolution Acts on the Phenotype Jessica Harwood Douglas Wilkin, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org AUTHORS Jessica Harwood Douglas Wilkin, Ph.D. EDITOR Douglas Wilkin, Ph.D. CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2015 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: November 23, 2015 CONTRIBUTORS Doris Kraus, Ph.D. Niamh Gray-Wilson Jean Brainard, Ph.D. Sarah Johnson Jane Willan Corliss Karasov www.ck12.org C HAPTER Chapter 1. Evolution Acts on the Phenotype 1 Evolution Acts on the Phenotype • Define carrier. • Explain how an unfavorable allele is kept in the gene pool. Would albinism be an advantage? This rabbit is albino, meaning it lacks pigment in its skin, fur, and eyes. The same thing happens in other species, including humans. To most animals albinism would be a disadvantage since they need to blend into their environment to avoid predators or catch prey. How, then, does the gene that causes albinism stay in the gene pool? Evolution Acts on the Phenotype Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, natural selection does not act on the underlying genotype (the genetic makeup) of an individual. For many traits, the homozygous genotype, AA, for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual. Even if the "aa" phenotype is lethal, the recessive a allele, will be maintained in the population through heterozygous Aa individuals. Furthermore, the mating of two heterozygous individuals can produce homozygous recessive (aa) individuals. However, natural selection can and does differentiate between dominant and recessive phenotypes. Carriers Since natural selection acts on the phenotype, if an allele causes death in a homozygous individual, aa, for example, it will not cause death in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers 1 www.ck12.org of the a allele, meaning that the a allele could be passed down to offspring. People who are carriers do not express the recessive phenotype, as they have a dominant allele. This allele is said to be kept in the population’s gene pool. The gene pool is the complete set of genes and alleles within a population. For example, Tay-Sachs disease is a recessive human genetic disorder. That means only individuals with the homozygous recessive genotype, rr will be affected. Affected individuals usually die from complications of the disease in early childhood, at an age too young to reproduce. The two parents are each heterozygous (Rr) for the Tay-Sachs gene; they will not die in childhood and will be carriers of the disease gene. This deadly allele is kept in the gene pool even though it does not help humans adapt to their environment. This happens because evolution acts on the phenotype, not the genotype ( Figure 1.1). FIGURE 1.1 Tay-Sachs disease is inherited in the autosomal recessive pattern. Each parent is an unaffected carrier of the lethal allele. Summary • Natural selection acts on the phenotype (the traits or characteristics) of an individual, not on the underlying genotype. • Carriers of a trait can show no symptoms of a recessive disease and, yet, still pass it on to their offspring. Explore More Use the resource below to answer the questions that follow. • Harmful Genes at http://www.newton.dep.anl.gov/askasci/mole00/mole00460.htm . 1. What would happen if a harmful gene were dominant? 2 www.ck12.org Chapter 1. Evolution Acts on the Phenotype 2. Give an example of a harmful recessive gene that provides carriers with an advantage. 3. How can a harmful gene "hide"? Review 1. What’s the difference between phenotype and genotype? 2. Does natural selection act on the genotype or phenotype? 3. Explain how a lethal recessive gene can stay in the gene pool. References 1. Zachary Wilson. Tay-Sachs disease, with autosomal recessive inheritance . CC BY-NC 3.0 3