Quantitative Genetics + The Evolution of Phenotypes (L14)

Exam 2 Lecture 14

Polygenic Trait

A trait influenced by multiple genes/genetic loci

• Heavily influenced by genetic and environmental factors

Ex: Height

Quantitative Genetics

The study of genetic mechanisms + evolution of continuous phenotypic traits

Phenotypic variation equation (V_P)

V_P = V_G + V_E

V_P = Total population variance

V_G = Variance due to genetic differences

V_E = Variance due to environmental differences

Genetic variation formula (V_G)

V_G = V_A + V_D + V_I

V_A = Additive variance

V_D = Dominance variance

V_I = Epistatic variance

Additive Genetic Variance (V_A)

The portion of genetic variance caused by the cumulative, individual effects of alleles on a trait

• responsible for the resemblance between parents and offspring

Dominance Genetic Variance (V_D)

The portion of phenotypic variance caused by the interaction of alleles at a specific locus, independent of the additive effects.

• Incomplete dominance - both alleles express dominance Ex: A₁A₁ is red, and A₂A₂ is white, then the offspring A₁A₂ is pink.

Epistatic Variance (V_I)

Interaction between alleles, but the alleles are associated with different loci

• The presence of a different allele may block the expression of another

Ex: With the previous flower example, if a pigment gene is B₁ for dominant and B₂ for recessive, and pigment is produced as long as the dominant allele is present, then the presence of a B₂B₂ genotype will turn off pigment, regardless of what the A allele is.

Heritability

a measure of the proportion of phenotypic variance (V_P) attributable to genetic variance (V_G)

• Broad sense heritability - H² = V_G/V_P

• Narrow sense heritability - h² = V_A/V_P

Environmental Variation Formula (V_E)

V_E = V_Eg + V_ExG + V_Es

V_Eg = general environmental variance

V_Es = specific environmental variance

V_ExG = genotype by environment interaction

Specific environmental variance (V_Es)

variance due to environmental conditions that are uniquely experienced by each individual

• considered the error or residual variance

General Environmental Variance (V_Eg)

nongenetic sources of variation experienced by multiple individuals in a population

Genotype by Environment interaction (V_ExG)

The different responses of genetic lines to general environmental variation

Selection differential (S)

The difference in the mean of a trait in reproducing individuals and the mean of the trait for the general population

• Measures the strength of selection

• R = Sh²