The genetic constitution of a population depends on many factors. The Hardy-Weinberg equilibrium is a concept, based on a mathematical equation, that describes the outcome of random mating within populations. It states that ‘in the absence of mutation, non-random mating, selection and genetic drift, the genetic constitution of the population remains the same from one generation to the next’.

This genetic principle has clinical significance in terms of the number of abnormal genes in the total gene pool of a population. The Hardy-Weinberg equation states that:

[l + 2pq + if = 1

where p is the frequency of the normal gene in the population, q is the frequency of the abnormal gene, [l is the frequency of the normal homozygote, if is the frequency of the affected abnormal homozygote, 2pq is the carrier frequency, and p + q = l.

This equation can be used, for example, to find the frequency of heterozygous carriers in cystic fibrosis: the incidence of cystic fibrosis is 1 in 2000 live births. Thus, if= 112000 therefore.

q = 1/44

and as p = 1 – q therefore

p = 43/44

The carrier frequency is represented by 2pq, thus, 1 in every 22 persons in the population is a heterozygous carrier for cystic fibrosis.