Unit 1 - quantitative genetics

Question 1

quantitative genetics

Answer 1

“methodology for doing statistical comparisons of genetic crosses”

study of the genetic mechanisms of continuous phenotypic traits

Question 2

key terms

Answer 2

variance:

broad sense heritability, H^2

narrow sense heritability, h^2

selection differential, S

Selection gradient, β

Selection response, R

Question 3

polygenic trait

Answer 3

influenced by many genetic loci
- interaction between alleles or among alleles
dominance: within loci or genes
epistasis: among loci or genes
-interaction with the environment
phenotypic plasticity

Question 4

Breeder’s equation and Darwin’s 5 steps

Answer 4

1. Individuals vary in appearance
   Vp > 0
2. Some of this variation is passed on from parent to offspring
   h^2 > 0
3. Excess offspring → struggle for existence
   Not Quantitative Genetics
4. Some individuals, due to their phenotypes, have higher survival &/or reproduction than others
   S ≠ 0, β ≠ 0
5. Those favored phenotypes increase in frequency in the next generation
   R ≠ 0

Question 5

Components of phenotypic and genetic variation

Answer 5

Phenotypic:
Vp = Vg + Ve
total phenotypic variance in pop. = variance due to genetic diff. + variance due to environmental diff.

Genetic:
Vg = Va + Vd + Vi
total genetic variance = additive + dominance + epistasis

Question 6

broad sense v narrow sense heritability

Answer 6

broad sense:
the proportion of phenotypic variance explained by genetic differences among individuals
H^2 = Vg / Vp = Vg / (Vg + Ve)

narrow sense:
the proportion of phenotypic variance explained by additive genetic variation (causes offspring to resemble parents)
h^2 = Va / Vp = Va / (Va + Vd + Vi + Ve)

Question 7

Modes of selection

Answer 7

directional: movement of the mean after selection (curve shifted)

stabilizing: reduction in variance after selection (skinnier curve)

disruptive: increase in variance (curve with two humps)

Question 8

Evolutionary response to selection

Answer 8

how much the population changes depends on:

-selection differential (S) (or selection gradient (B))
• S is the difference in mean phenotypes, after
minus before selection, within the parental
generation.
• β is the slope of the regression line of fitness
component (Y axis) versus phenotype (X axis)
within the parental generation
*selection differential measures the strength of selection

-heritability
*high heritability results in a larger change

Question 9

the breeder’s equation: calculating the evolutionary response to selection

Answer 9

• R is the difference in mean phenotypes between the offspring generation (after selection on the parents) minus the parental generation before selection on the parents.

• R = h2 x S
– Used in selective breeding in agriculture, and inartificial selection experiments

• R = VA x β
– Used in field experiments, where fitness versus phenotype usually is not all or none, but continuous

Question 10

key concepts:

Answer 10

• evolution and selection are not the same
– Selection can occur without evolution
– Selection happens within a generation, whereas, evolution happens between generations

• The magnitude of change depends on:
– Strength of selection (selection differential)
– Heritability