# Quantitative Genetics Flashcards

What are complex traits?

Measures of an individual that vary in degree rather than kind

Eg. growth rate, height, weight

Study of the genetics known as quantitative genetics

Why are complex traits important?

Evolution - much adaptation results from natural selection operating on quantitative traits, Eg. beak size in Darwin’s finches

Animal & plant breeding - target for artificial selection in domesticated animals and crops

Human medicine - many health problems results from diseases whose susceptibility varies on a continuous scale, Eg. BMI predicts cancer risk

What are the fundamental properties of complex traits?

Resemblance between relatives, share alleles inherited identical by descent, may share same genotype at some loci, tend to be exposed to similar environments

Respond to selection, artificial and natural, offspring from selected population tend to score higher than offspring of an unselected population, means changes across generations

Show inbreeding depression and hybrid vigour, inbreeding reduces fitness of offspring which can be restored by crossing with unrelated strain (hybrid vigour), changes frequency of homozygotes

Many show a continuous distribution

What are the causes of variation for complex traits?

Genetic - alleles at many loci affecting the trait segregate simultaneously, each allele at each locus affecting the trait generally has small effect, infinitesimal model

Environmental - increase/decrease trait, blur distinctions between genotypes

P = G + E

How can continuous variation arise from many loci?

Assume several loci affect a trait each with two alleles

At each locus each alleles of type 1 adds one unit to trait value

Heterozygote is exactly intermediate between homozygotes - additive gene action

Assume allele frequencies are same for each locus and genotypes segregate in HW proportions

I there’s 1 locus each genotypes has unique phenotype, only 3 phenotypes, if 2 loci 9 genotypes and 5 phenotypes, intermediate phenotypes generated by several genotypes

With increasing loci becomes more like a normal distribution

What are the measurable properties of complex traits?

If we directly measure an individual for a trait we obtain its phenotypic value

If we measure a sample of individuals in a population can estimate some statistical properties of the quantitative trait - mean, variance, covarience

What is variance?

A measure of the dispersion of the distribution of values

What is covariance?

A measure of the association between pairs of values

How do you calculate variance?

(sum of (difference of each value)^2) / n-1

Divide by n-1 as you always overestimate variance in a sample of population

How do you calculate covariance?

(sum of (difference of each X value * difference of each Y value)) / n-1

How can genetic variation be quantified?

The amount of phenotypic resemblance among relatives for a quantitative trait can be used to quantify the amount of genetic variation for the trait

How can phenotypic resemblance between relatives be measured?

The degree of similarity among a group of relatives for the trait compared to random members of a population (covariance of family members)

Equivalent to

Extent of differences in phenotype between different families (variance between families)

What is a calculation to quantify resemblance between relatives?

P = G + E

P - phenotypic value

G - genotypic value

E - environmental value

What do we assume for the environmental value (E)?

Usually assumed not to be transmitted to offspring

Mean assumed to be zero

How can we estimate the genotypic value (G)?

If you raise multiple cloned progeny from a parent in random environments, G, of the parent estimated as the mean phenotypic value (P) of the progeny BUT not a likely scenario

How do we model the components of the genotypic value (G)?

G = A + D

G - genotypic value

A - breeding value, transmitted from parent to offspring, individual judged by the average value of its offspring

D - dominance deviance, interactions between pairs of alleles not transmitted from parent to offspring

G is not wholly transmitted from parents to offspring as alleles and not genotypes are transmitted across generations

How can we measure breeding value (A) of an individual?

Mate one male to random unmated females and raise offspring in multiple, random environments

Difference between mean offspring’s phenotypic value and population mean in half male parent’s breeding value (A/2)

What is heritability of a trait?

The proportion of the phenotypic variation for a quantitative trait that’s genetic

What can we predict from the heritability?

Resemblance between relatives

Response to selection

What are the 2 measures of heritability?

Broad sense heritability (H^2): for clonal organisms, proportion of phenotypic variance caused by variation among individuals in their genotypic values (G)

H^2 = Vg/Vp

Narrow sense heritability (h^2): diploid organisms, proportion of phenotypic variance caused by variation among individuals in their breeding values, A:

h^2 = Va/Vp

Which measure of heritability is more important?

Narrow sense heritability

Directly determines parent-offspring resemblance and response to selection

So need to estimate Va to estimate heritability

Vp easy to estimate as it’s the variance of phenotypic values

How is the variance in breeding values (Va) calculated?

Mate male parents to multiple, randomly chosen females from a population to generate several large half-sib families

Dominance and environmental effects assumed not to be transmitted

Do this for multiple males and find the variance

Variance of these family deviations from the population mean = Vhs = Va/4

What does heritability not indicate?

Doesn’t indicate absolute amount - can only be compared across traits, scales, populations

Doesn’t indicate number of genes

Low heritability doesn’t mean not genetically determined, just has a low genetic variance

What are the 2 steps to finding heritability interference?

Determine arithmetic relationship between heritability and covariance between particular relatives we are interested in

Work out a way to statistically estimate covariance