Natural Selection: Hardy-Weinberg Principle (Cambridge (CIE) A Level Biology) : Revision Note

Natural selection: the Hardy-Weinberg principle

• The Hardy-Weinberg principle can be used to predict allele frequencies in a population

• The principle can only be applied accurately to populations under certain conditions, e.g. in populations where:

  • no natural selection is taking place

  • there is no migration into or out of the population

  • mating is random

  • the population is large

  • no mutations are occurring

Hardy-Weinberg equations

• If the phenotype of a trait in a population is determined by a single gene with only two alleles (we will use B / b as examples throughout this section) then the population will consist of individuals with three possible genotypes:

  • Homozygous dominant (BB)

  • Heterozygous (Bb)

  • Homozygous recessive (bb)

•  When using the Hardy-Weinberg equation, frequencies are represented as proportions of the population; a proportion is a number out of 1

• The frequency of alleles can be represented; this is the proportion of all of the alleles in a population that are of a particular form 

  • The letter p represents the frequency of the dominant allele (B)

  • The letter q represents the frequency of the recessive allele (b)

  • As there are only two alleles at a single gene locus for a phenotypic trait in the population:

p + q = 1

• E.g. in a population of 100 individuals there would be 200 alleles because every individual has two versions of each gene

  • If 120 of those alleles were the dominant allele then the frequency of the dominant allele would be 120/200

  • It could be said that p = 120 ÷ 200 = 0.6

  • If p = 0.6 then q = 1 - 0.6 = 0.4

• The frequency of genotypes can also be represented; this is the proportion of all of the individuals with a particular genotype

  • The frequency of homozygous dominant individuals is represented by p²

  • The frequency of heterozygous individuals is represented by 2pq

  • The frequency of homozygous recessive individuals is represented by q² 

  •  As these are all the possible genotypes of individuals in the population the following equation can be constructed:

p² + q² + 2pq = 1