(3) WJEC A2 Biology 4.4 Population genetics

Gene pool

The total of all the alleles of all the genes in a population at a given time

Population genetics is not concerned with the genotypes of individuals, but describes the allele frequencies.

Define allele frequency

It is the allele’s proportion/fraction/percentage out of all the alleles of that gene in the gene pool.

Genetic drift

Chance variations in allele frequency in a population. Most significant in small or isolated populations when a small number of alleles form a large proportion of the total.

For example, in a very small population, it may be that, by chance, none of those possessing a certain allele mate with one another. Then the allele is lost from the population altogether!

Hardy-Weinberg principle

States that, in ideal conditions, allele and genotype frequencies are constant from generation to generation. There are 9 ideal conditions

The conditions of the Hardy-Weinberg principle

Organisms:

• Are diploid

• Allele frequencies are equal in both sexes

• Reproduce sexually

• Mating is random

• Generations don’t overlap

Population:

• Very large

• No immigration/emigration

• No mutation

• No selection

Hardy-Weinberg equation

p² + 2pq + q² = 1

• p is frequency of the DOMINANT allele

• q is frequency of the RECESSIVE allele

thus:

• p² is dominant homozygous frequency (e.g AA)

• 2pq is heterozygous frequency (Aa)

• q² is recessive homozygous frequency (aa)

Under the 9 conditions of the HW principle, p and q frequencies remain constant so the population is said to be in..?

Hardy-Weinberg equilibrium