Question

The fitness of the A1A1, A1A2, and A2A2 genotypes are 1.2, 0.9, and 1.0, respectively. Calculate the selection coefficient and heterozygous effect. What kind of selection is this? Where will the population ultimately end up in terms of allele frequencies?

Answer 1

Relative fitnesses of the genotypes with respect to that of A2A2 are the same as their
absolute fitnesses: 1.2/ 1.0 = 1.2, 0.9/ 1.0 = 1.1 and 1.0 / 1.0 = 1.0 for A1A1, A1A2, and A2A2,
respectively. However, with respect to the most fit genotype AA, the relative fitnesses are
1.2 / 1.2 = 1, 0.9/ 1.2 = 3/4, and 1.0 / 1.2 = 5/6 for A1A1, A1A2, and A2A2, respectively.

Selection coefficient (s) and the heterozygous effect:

if the relative fitness of the fittest homozygote is defined as 1, then we define s as the
difference between that and the relative fitness of the other homozygote:
s = 1-5/6=1/6

The heterozygous effect can be found from the fitness of the heterozygote:
3/4 = 1 - hs
hs = 1 – 3/4=1/4
h = 6*1/4=6/4
h =3/2=1.5
This is a case of underdominance, since h>1.