BB 3140 Course Outline Chapter 9 B 2009

BB 3140 Course Outline Chapter 9 B 2009 - BB 3140 Course...

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BB 3140 Course Outline Chapter 9 Variation in Populations I. Phenotypic variation can be genetic or environmental (Fig. 9.2) A. Genetic variation 1. Can be simple (one locus) a. a single allele pair (Fig. 9.2A) b. multiple alleles at a single locus (Fig. 9.2AB 2. Can be complex (multiple loci) a. more than one locus can control a characteristic independently b. Quantitative traits i. many different loci contribute quantitatively to a phenotype (Fig. 9.2C) 3. How to tell if phenotypic variation is genetic, environmental, or a combination a. Phenotypes should follow a Mendelian segregation pattern: 3:1 or 1:2:1 ratio in crosses, if variation is genetic b. More closely related individuals should resemble each other more than they do unrelated individuals, if variation is genetic c. environmental differences as a cause of variation can be eliminated in several ways i. study unrelated individuals that share the same environment with the related individuals in question (e.g., study adopted children) ii. rear offspring of phenotypically different individuals in a “common garden” (uniform environment) II. Principles of Genetic Variation in Populations A. At a given locus, a population may contain two or more alleles that differ in DNA sequence and confer different phenotypes. In a diploid, each individual contains only two alleles at this locus. B. Two important definitions (illustrated in Fig. 9.6) 1. Allele frequency: the proportion (fraction) of a given allele found in a population. Measured by counting all the alleles of a given type in a population and dividing by the total number of alleles sampled. 2. Genotype frequency: The proportion of individuals in a population that has a certain genotype at a locus C. Fundamental concepts 1. Alterations in allele frequency in one generation will result in altered frequencies in the next generation. 2. Such alterations represent the result of evolutionary change 3. Factors that change allele frequencies are the causes of evolution
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D. Hardy-Weinberg equilibrium (see derivation in box 9A, p. 222, example in Fig. 9.7) 1. Genotype and allele frequencies remain constant from one generation to the next, if random mating occurs
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BB 3140 Course Outline Chapter 9 B 2009 - BB 3140 Course...

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