Quantitative Genetics and Evolution of Phenotypes

polygenetic traits

aka quantitative genetic traits

show continuous rather than discrete variation

influenced by many genetic loci

-interaction between alleles (epistasis)

-interaction with environment (phenotypic plasticity)

tend to vary continuously among individuals

Quantitative genetics

study of the genetic mechanisms and evolution of continuous phenotypic traits

variance

measure of how widely dispersed trait values are from the mean

Vp

total variance in a phenotypic trait in a population

Vg

variance due to genetic differences

Ve

variance due to environmental differences

Ve > Vg

environmental factors play bigger role in phenotypic variations than do alleles

Vp ~ Vg

environmental factors negligible

phenotypic varation eq

Vp = Vg + Ve

*Vg = Va + Vd + Vi

Va = additive genetic variance

Vd = dominance effects of alleles on variance

Vi = variance due to epistatic interactions among alleles

Broad sense heritability (H^2)

proportion of total phenotypic variation of a trait that is attributable to genetic differences among individuals

H^2 = Vg/Vp = Vg/Vg+Vp

Narrow sense heritability (h^2)

proportion of phenotypic variance explained by additive effects of alleles

h^2 = Va/Vp = Va/Vg+Ve = Va/Va+Vd+Vi+Ve

*causes offspring to resemble their parents and it causes populations to evolve in a predictable way in response to selection

Breeder's Equation

R = h^2 x S

R = evolutionary response

h^2 = heritability (narrow sense)

S = selection differential

*how to predict whether quantitative trait will evolve in response to selection

S

Selection differential

measures strength of selection

(mean trait value of reproducing individuals) - (mean trait value of entire population)

R

evolutionary response

high heritability results in larger change

(mean trait value of offspring) - (mean trait value of original population)

product of strength of selection (S) and heritability of trait

linkage disequilibrium

exists if the occurrence of an allele at one locus is nonrandomly associated with the presence or absence of an allele at a second locus

-Two different loci close together on the same chromosome can be nonrandomly linked

how to find the genes responsible for a heritable polygenic trait

SNPs

genetic markers that can be easily

measured and mapped to specific locations along

the chromosomes

-dr screen for this as cheap way to view disease causing allele its linked to bc disease harder to locate

Quantitative Trait Locus (QTL) analysis

links traits with genes

stretches of DNA that are correlated with variation in a phenotypic trait

Genome-wide association (GWA) studies

Instead of crossing individuals from genetically divergent populations (as we saw with QTL studies), GWA studies use a large number of individuals with and without a trait (disease).