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Lecture

BIOB10Y3 Lecture Notes - Phenotypic Trait, Quantitative Genetics, Mendelian Inheritance


Department
Biological Sciences
Course Code
BIOB10Y3
Professor
C

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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
oSuch as height, athletic ability, and intelligence
oAlso 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
oMeasurements must be taken
oCharacters 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:
oIf 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 (Vp) into a component due to genetic
variation (VG) and a component due to environmental variation (VE)
Heritability- fraction of the total variation in a trait that is due to variation in genes
oIn 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 (VP)- total variation in a trait
Genetic Variation (VG)- variation among individuals that is due to variation in their genes
Environmental Variation (VE)- variation among individuals due to variation in their environments
Broad-Sense Heritability or Degree of Genetic Determination:
Heritability = VG/ VP = VG/ (VG + VE)
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 (h2)- that fraction of the total phenotypic variation in a population that is due to
the additive effects of genes
oIs an estimate of the fraction of the variation among the parents that is due to variation in their genes
h2= VA/VP = VA/ (VA + VD + VE)
Heritability, h2, is a measure of the (additive) genetic variation in a trait
Additive genetic variation (VA) is variation among individuals due to the additive effects of genes
Dominance genetic variation (VD) is variation among individuals due to gene interactions such as
dominance
ototal genetic variation is the sum of the additive and dominance genetic variation
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