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Lecture

Quantitative Genetics

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Department
Biology
Course
BIOLOGY 1P03
Professor
Douglas Davidson
Semester
Fall

Description
Chapter 9- Quantitative Genetics • Quantitative Genetics- the branch of evolutionary biology that provides tools for analyzing the evolution of multi-locus traits 9.1- The Nature of Quantitative Traits • Qualitative Traits- characteristics that we can assign to individuals by just looking at them, or perhaps by conducting a simple genetic test • Traits with discrete phenotypes are special examples; most traits in most organisms show continuous variation o Such as height, athletic ability, and intelligence o Also beak length in soapberry bugs and bill depth in medium ground finches • Traits with continuous variation cannot assign individuals to discrete phenotypic categories by simple inspection o Measurements must be taken o Characters with continuously distributed phenotypes are called quantitative traits  Are determined by the combined influence of: • Genotype at many different loci • & The environment • Quantitative traits are traits for which the distribution of phenotypes is continuous rather than discrete • Quantitative traits are consistent with Mendelian genetics. They are influenced by the combined effects of the genotype at many loci. Quantitative traits are also influenced by the environment. 9.3- Measuring Heritable Variation • Basic tenets of Darwin’s theory of evolution by natural selection: o If there is heritable variation among the individuals in a population, and if there are differences in survival and/or reproductive success among the variants, then the population will evolve. • Quantitative genetics includes tools to measure heritable variation, tools for measuring differences in survival and/or reproductive success, and tools for predicting the evolutionary response to selection • Quantitative genetics allows us to analyze evolution by natural selection in traits controlled by many loci • The first step in a quantitative genetic analysis is to determine the extent to which the trait in question is heritable. That is, we must partition the total phenotypic variation (V p into a component due to genetic variation (VG) and a component due to environmental variation (V ) E • Heritability- fraction of the total variation in a trait that is due to variation in genes o In the broad sense, that fraction of the total phenotypic variation in a population that is caused by genetic differences among individuals; in the narrow sense, that fraction of the total variation that is due to the additive effects of genes. • Phenotypic Variation (V )- tPtal variation in a trait • Genetic Variation (V )- Gariation among individuals that is due to variation in their genes • Environmental Variation (V )- vaEiation among individuals due to variation in their environments • Broad-Sense Heritability or Degree of Genetic Determination: Heritability = V G VP= V /G(V G V ) E Estimating Heritability from Parents and Offspring • If the variation among individuals is due to variation in their genes, then offspring will resemble their parents • Figure 9.13: o [9.13a] If offspring do not resemble their parents, then the slope of the best-fit line through the data will be near -; this is evidence that the variation among individuals in the population is due to variation in their environments, not variation in their genes o [9.13c] If offspring strongly resemble their parents, the slope of the best-fit line will be near 1; this is evidence that variation among individuals in the population is due to variation in their genes, not variation in their environments o [9.13b] Most traits in most populations fall somewhere in the middle, with offspring showing a moderate resemblance to their parents; this is evidence that the variation among individuals is partly due to variation in their environments and partly due to variation in their genes • Narrow-sense Heritability (h )- that fraction of the total phenotypic variation in a population that is due to the additive effects of genes o Is an estimate of the fraction of the variation among the parents that is due to variation in their genes 2 h = V AV P V /A(V +AV + D ) E • Heritability, h , is a measure of the (additive) genetic variation in a trait • Additive genetic variation (V ) is variation among individuals due to the additive effects of genes A • Dominance genetic variation (V ) is vDriation among individuals due to gene interactions such as dominance o total genetic variation is the sum of the additive and dominance genetic variation  V G V +AV D • Narrow sense heritability, h , allows us to predict how a population will respond to selection Estimating Heritability from Twins • Monozygotic (id
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