L17 Quantitative Genetics 2

Question 1

For complex quantitative traits

Answer 1

• degree of resemblance within families is a function of the genetic basis of the trait
• but the phenotypic value of the offspring is NOT exactly equal to the mean value of the parents
• the resemblance between relatives can be formally calculated under the additive quantitative genetics model

Question 2

Level of relatedness

Answer 2

level of relatedness between individuals is expected to lead to some trait similarity

Question 3

Genetic covariance

Answer 3

trait similarity between relatives is formally calculated in terms of genetic covariance

Question 4

Heritability

Answer 4

proportion of genetic variance compared to the total variance is called heritability

• makes strong assumptions: linear decomposition of effects (G and E) and no covariation/interaction among them
• strictly population specific, which means that any change in allele freq change its value

Question 5

Response to selection

Answer 5

knowing the level of heritability allows us to predict the response to selection of a population across generations

Question 6

Variance

Answer 6

variance of a variable is a measure of the deviation of the observed values from the expected mean
- genetic values being fixed, genetic variance is due to randomness of allele frequency

Question 7

Covariance

Answer 7

covariance of two variables is a measure of the joint variability of two variables e.g. how the two variables deviate jointly from their expected mean
- in genetics, the covariance between individuals for a trait is directly proportional to their level of relatedness

Question 8

Degree of identity

Answer 8

IBD - estimated from pedigree of individual

IBS - determined from sequence data

Question 9

Covariance between parent and offspring

Answer 9

See onenote

Question 10

Covariance between genetic effects

Answer 10

see onenote

Question 11

Covariance between sibs

Answer 11

see onenote

Question 12

covariance between any relatives

Answer 12

see onenote

Question 13

Partitioning the phenotype variance among and within families

Answer 13

see onenote

ANOVA

covariance between individuals of a family is by definition the variance among families

Question 14

Broad-sense heritability

Answer 14

ratio of genetic variance and the total phenotypic variance e.g. metric of how much trait variation is due to genetics

Question 15

Narrow-sense heritability

Answer 15

how much of the phenotype variance can be immediately transferred to the next generation

Question 16

Experimental designs to measure heritability

Answer 16

1. identical twins or clones = provides an estimate within one generation using ANOVA-type framework
2. comparisons between pairs of parent and offspring = provides an estimate over two generations using a regression framework
3. response to selection = provides an estimate over two generation with very little effort (only based on the means before, during and after selection)

Question 17

Pairs of monozygotic twins

Answer 17

• provides direct estimate of total genetic variance

- no decomposition into additive and dominance effects, thus of limited interest to breeders

Question 18

Midparent/offspring regression

Answer 18

see onenote

• covariance between offspring and parent provides direct estimate of heritability
• parents only directly transfer additive value to offspring. dominance effect calculation requires precise knowledge of IBD probability
• covariance of P/O is thus an estimate of additive variance, leading to an estimate of narrow sense heritability

Question 19

Response to selection

Answer 19

see onenote

Question 20

Breeder’s equation

Answer 20

predicts how a population will respond to selection under the hypothesis of linear selection

Question 21

The heritability of a trait depends on

Answer 21

• the genetic architecture of the trait (number of loci, genetic effect, additivity/dominance)
• freq. of genotype in a population