Genetics - quantitative traits and population genetics

What are quantitative traits?

Traits that vary continuously - height, weight, etc

What determines the expression of quantitative traits?

• Controlled by the effects of multiple genes and alleles

• Also affected by environmental influences

What are the three types of polygenic traits?

• Metric - continuous scale

• Meristic - discrete scale

• Threshold - present or absent beyond a certain value

• All three are based on the theoretical assumption of an underlying normal distribution

What fields are quantitative traits particularly important in?

• Medicine

• Agriculture

• Conservation

How are quantitative traits important in medicine?

• Susceptibility to disease

• Complex disorders caused by multiple genetic and environmental factors

• Understanding genetic vs environmental causes

• Prevention of diseases

• Genetic counselling

• Genetically tailored treatments

How are quantitative traits important in agriculture?

• Economically important traits are quantitative

• Quantitative genetics theory and the basis for selective breeding

• Environmental variation reduces efficiency of selection

What factors contribute to phenotypic variation?

Vp = Vg + Ve + Vge

• Vp - phenotypic variation

• Vg - genetic variation

• Ve - environmental variation

• Vge - genetic + environmental interaction variation

What factors contribute to genetic variance?

Vg = Va + Vd + Vi

• Vg - genetic variance

• Va - variance due to additive effects

• Vd - variance due to dominance

• Vi - variance due to epistasis

What is variance due to additive effects (Va) and how does it affect variance?

• Portion of genotypic variance that results from the additive effects of alleles where each allele contributes independently

• Each allele’s effect on the phenotype does not depend on any other allele within the gene or in another gene

What is variance due to dominance effects (Vd) and what effect does it have on variance?

• Portion of genotypic variance that results from the non-additive interaction between alternative alleles in the same gene

• Each allele’s effect on the phenotype depends on the other alleles within the gene

What is variance due to epistasis (Vi) and what effect does it have on variance?

• Portion of phenotypic variation that results from the non-additive interaction between different genes

• Each allele’s effect on phenotype depends on genotype in other genes

How does meiosis change associations between alleles?

• Can make it difficult to select for phenotypes that rely on dominance and epistatic effects

• Additive genetic effects on the phenotype are independent of other alleles so recombination will not break up favourable allele combinations within and between genes

What are the two types of heritability?

• Broad sense heritability

• Narrow sense heritability

What does broad sense heritability measure?

• Measures the importance of genetic variation relative to total variation in the phenotype

• Tells how much is related to genetics and how much to the environment

What does narrow sense heritability measure?

• Measures the importance of additive genetic variation or variation due to the alleles in the population relative to total variation in phenotypic variation

• Tells us how much is additive and how much is environmental

What can heritability be used to predict?

• To predict change in the population mean under selection (natural or artificial)

• When h² increases the response to selection increases

How can heritability vary?

• Changes when a population is moved between environments

• Heritability for the same trait can vary between populations even if they are in the same environment

What is the Infinitesimal Model?

• Simple model of the inheritance of quantitative traits which assumes an infinite number of unlinked loci with an infinitesimal effect each

• Infinite number of genes controlling a phenotypic trait each with a very small effect

What does population genetics theory state?

• The frequency of alleles and genotypes in a population will remain constant unless acted upon by mechanisms of evolution

• Equilibrium = no evolution

What is the Hardy-Weinberg equation for two alleles?

p + q = 1

• p - frequency of one allele (usually dominant)

• q - frequency of another allele (usually recessive)

p² + 2pq + q² = 1

• p² - proportion of population homozygous for the first allele

• 2pq - proportion of population that is heterozygous

• q² - proportion of population homozygous for the second allele

What are the assumptions of the Hardy-Weinberg equilibrium?

• Organisms are diploid, mating and have discrete generations

• Allele frequencies are the same in each individual

• Mendelian segregation occurs

• Mating occurs at random

• Population size is large so has no genetic drift

• No gene flow by immigration or emigration

• No mutation

• No selection

What is linkage disequilibrium?

• Deviations from the expectations of independent segregation and Hardy-Weinberg caused by physical linkage or population demography

• eg genetic bottleneck events