Quantitative Genetics Flashcards
Heritability (H^2)
The proportion of total variation in the population that is due to genetic variation
In genetic terms, this is the proportion of total phenotypic variance, which is due to genetic variance:
(H^2)=Vg/Vp
Heritability (H^2) is a ratio depends on both genes and environment.
Unless one is held constant, high heritability could be due to shared genes, shared environments, or both.
Need to be very clear about the limits of what heritability means. On farms, we can hold the environment constant (therefore any differences between individuals would be due to variation in genes), but this is much harder for humans.
Phenotypic variance (Vp)
The total phenotypic variance (VP) is a result of variance caused by the environment (VE), the genes (VG) and the interaction of the gene and environment (VGE), in other words:
VP = VE + VG + VGE
As phenotypic variance includes both gene and environment and gene-environment interaction, this is equivalent to:
H2 = VG / VG + VE + VGE
H2 is the broad sense heritability
The total genetic variance can be attributed to three distinct sources:
Additive genetic variance (Va): variance associated when the addition of an allele contributes a fixed value to the value quantitative trait: ++ = 2cm; +- = 1cm; – = 0cm
Dominance genetic variance (Vd): variance caused by the masking of recessive alleles by dominant
alleles in the heterozygote: ++ = 2cm; +- = 2cm; – = 0cm
Interaction genetic variance (Vi): variance caused by the interaction between different loci: ++ ++ = 2cm; ++ +- = 1.2cm; ++ – = 0cm
Narrow sense heritability
Typically plant and animal breeders are only interested in the variance caused by additive genetic variance because this can be directly selected for. NARROW SENSE HERITABILITY (h2) is given by:
h^2 = Va / VP
Narrow sense heritability has not been measured in any human traits. Traits in chickens can have
high values for narrow sense heritability (such as weight), but not yolk weight.
Heritability of horse race speed. On a dry track, a controlled environment, most of the variation in
speed is due to genes, so value of h2 is high. But on a wet less controlled racecourse the value of h2 will reduce.
How to estimate heritability
Use the similarities of relatives - need to measure correlation
Example of estimation
Now, imagine that these men have sons whose height is:
Check doc
Work out the way in which fathers and sons tend jointly to deviate from the average. For example, if mean height in group studied is 1.6 for fathers and 1.8 for sons, father 1 is 1.4m and son 1 is 1.6m. Covariance for this point is (1.4 - 1.6)(1.6 -1.8) which is -0.2 x -0.2 = 0.04. There seems to be a fit between the height of the parents and offspring, presumably evidence of their heritability.
Twin studies quantitave genetics
Twin studies. Monozygotic (MZ) twins share all their genes (genetic clones) and dizygotic (DZ) twins have half their genes in common.
The argument is that given both MZ and DZ twins share an environment, variation between the sets of twins will be genetic. This is called concordance; extent to which twins share the same phenotype.
Concordance values for traits such as diabetes and obesity are higher in MZ twins than DZ twins.
There are problems with this argument. The assumption of these studies is that the environment shared by twins is the same for both twins both before and after birth. But this is not the case.
Even before they are born, there is variation of when the splitting of the embryo occurs causes variation in MZ twins.
After birth, MZ twins are more likely to copy each other and their parents more likely to
raise them more similarly (such as dressing them the same) than in DZ twins.
Monochorionic twins share the same blood supply whilst DZ twins do not, which means they are competing for nutrients. MZ twins can therefore differ significantly at birth.
