Lecture 12: Quantitative Trait Analysis Flashcards
(21 cards)
how can we determine how much variation is attributable to genetic or environmental variation?
we can separate the genetic effects from the environmental effects by quantifying one variable, while controlling the other
draw a graph of stem length vs number of plants, and label the mean and variation
equation for mean
equation for variance
Ve
environmental variance - the portion of phenotypic variation in a population that is due to environmental factors, rather than genetic factors
Vg
genetic variance - a measure of the variation in a trait due to differences in genotype within a population
Vp =
Vg + Ve
Broad sense heritability of a trait
the extent of phenotypic variation that is attributable to genetic variation
H^2 = Vg/Vp
describe what different values of H^2 could mean
H^2 = 1
- all the phenotypic variation is attributable to genetic variation
H^2 = 0
- all the phenotypic variation is attributable to environmental effects
how and why is the heritability of a trait defined?
- heritability of a trait is always defined for a specific population or specific family in a specific set of environmental conditions
- this is because the amounts of genetic, environmental, and phenotypic variation may differ among traits, families, populations and among different environments
what does H^2 tell us? what does H^2 NOT tell us?
Tells Us:
If H^2 is high, the phenotype of an individual is likely to be attributable to its genotype IN THAT FAMILY
Does not tell us:
1. What phenotype an individual will have based on their parents’ phenotypes. Even if H^2 is high, an individual’s precise phenotype cannot be predicted based on its parents’ phenotypes (parents pass on their alleles, not their genotype)
2. What is going on in other families. H^2 is family specific and varies among different families, populations, or environments
why is H^2 not predictive?
Vp = Vg + Ve
however,
Vg = Va + Vd + Vi
where
- Va is variation due to additive effects
- Vd is variation due to dominance effects
- Vi is variation due to epistatic effects
Vd
dominance variance, which is the portion of genetic variance attributable to interactions between alleles at the same locus
is Vi predictive?
epistatic interaction variation is not transmitted from parents to offspring -> new genotypes and thus new epistatic relationships are formed with each generation
if a trait is due to additive effects, is it predictive?
yes
narrow-sense heritability
the extent of phenotypic variation that is attributable to additive genetic variation
describe different h^2 values and what they can mean
if all the phenotypic variation is attributable to additive variation, h^2 = 1 (the maximum it can be)
if all the phenotypic variation is attributable to other genetic and environmental effects, h^2 approaches 0
h^2 =
Va/Vp
use bill depth in Darwin’s finches as an example of narrow sense heritability
~ 18% of variation is attributed to the environment and or dominant or epistatic genetic variation
slope = 0.82
~ 82% of variation is attributed to additive genetic variation
what does h^2 tell us vs not tell us?
tells us:
if h^2 is high, the phenotype of an individual is predictable based on the phenotype of its parent in that family
does not tell us:
what is happening in other families
how can twins be used to study heritability of polygenic traits?
- monozygotic twins: single ovulated egg fertilised by one sperm, then the embryo splits into two. 100% of alleles are shared, so we can study the effect of different environments on the same genotype
- dizygotic twins: two ovulated eggs fertilised by different sperm, so 50% of alleles are shared, so we can study the effect of different genotypes on the same environment
