3.7.2 Populations

As we know from 3.7.4, a population is a group of organisms of the same species in a particular area at a particular time that can potentially interbreed. All the alleles of all the genes of all the individuals in a population at a given time are known as the gene pool. The number of times an allele occurs within the gene pool is referred to as the allelic frequency.

The Hardy–Weinberg principle provides a mathematical model, which predicts that allele frequencies will not change from generation to generation. The principle makes an assumption that the proportion of dominant and recessive alleles of any gene in a population is constant from one generation to the next. This can be the case if the following conditions are met:

No mutations arise

The population is isolated (there is no flow of alleles into or out of the population)

There is no selection pressure (all alleles are equally likely to be passed onto the next generation)

The population is large

Mating with the population is random

A good way of understanding the principle is to look at a gene that has two alleles, a dominant one (A) and a recessive one (b)

Let the probability of allele A = p

Let the probability of the allele a = q

We can write two equations from this. The first is: 

p + q = 1.0

This is because the probability of one plus the other must be 100% (1.0)

There are only four arrangements of the two alleles, AA + Aa + aA + aa. The probability of all four added together must equal 100% (1.0). It follows that…

p² + 2pq + q² = 1

where p is the frequency of the dominant allele and q is the frequency of the  recessive allele of the gene.