Question

How does Hardy-Weinberg’s expression (p²+2pq+q²=1) explain that genetic equilibrium is maintained in a population? List any four factors that can disturb the genetic equilibrium.

Answer 1

The allele frequencies in a population are stable and are constant from generation to generation in the absence of gene flow, genetic drift, mutation, recombination, and natural selection. If a population is in a state of Hardy Weinberg equilibrium, the frequencies of alleles and genotypes or sets of alleles in that population will remain the same over generations. Evolution is a change in the allele frequencies in a population over time. Hence population in Hardy Weinberg is not evolving.

Suppose we have a large population of beetles, (infinitely large) and appear in two colours’ dark grey (black) and light grey, and their colour is determined by the ‘A’ gene. ‘AA’ and ‘Aa’ beetles are dark greys and ‘aa’ beetles are light grey. In a population let’s say that the ‘A’ allele has a frequency (p) of 0.3 and the ‘a’ allele has a frequency (q ) of 0.7. Then p + q = 1.

If a population is in Hardy Weinberg equilibrium the genotype frequencycan be estimated by Hardy Weinberg equation.
(p + q)² = p²+ 2pq + q²
p² = frequency of AA
2pq = frequency of Aa
q² = frequency of aa
p = 0.3, q = 0.7 then,
p² = (0.3)² = 0.09 = 9 %AA
2pq = 2(0.3) (0.7) = 0.42 = 42 % Aa
q² = (0.7)² 0.49 = 49 % aa

Hence the beetle population appears to be in Hardy - Weinberg equilibrium. When the beetles in Hardy - Weinberg equilibrium reproduce the allele and genotype frequency in the next generation would be: Let’s assume that the frequency of ‘A’ and ‘a’ allele in the pool of gametes that make the next generation would be the same, then there would be no variation in the progeny. The genotype frequencies of the parent appear in the next generation. (i.e. 9% AA, 42% Aa and 49% aa).

If we assume that the beetles mate randomly (selection of male gamete and female gamete in the pool of gametes), the probability of getting the offspring genotype depends on the genotype of the combining parental gametes.