Introducing the Hardy-Weinberg principleThe Hardy-Weinberg principle is a mathematical model used to calculate the allele frequencies of traits with dominant and recessive alleles.

The model assumes that the population:

is large has random mating is experiencing no selection has no mutation, emigration or immigration.

If these assumptions are met then the allele frequencies of the population will remain stable over time.

The Hardy-Weinberg principle

Calculating allele frequencies

Factors affecting the gene pool

The Hardy- Weinberg Principle

• Principle predicts the frequencies of alleles in a population wont change from one generation to the next.

• But only when:There is a large populationNo immigration or emigrationNo mutationsNo natural selection.

So why is it useful?

• Estimates the frequency of particular alleles and genotypes within populations.

• Can be used to test whether or not the Hardy-Weinberg principle applies to particular alleles in particular populations, i.e. To test whether selection or any other factors are influencing allele frequencies.

Predicting Allele Frequency

p + q =1

p= the frequency of the dominant allele q= the frequency of the recessive allele

The total frequency of all possible alleles for a characteristic in a certain population is 1, so the frequencies of certain alleles must add up to 1.

Example

• A species has either red or white flowers. Allele R (red) is dominant and allele (white) is recessive.

• If the frequency of R is 0.4, then what is the frequency of r?

Genotype frequency

P2 + 2pq + q2 = 1

P2 = frequency of homozygous dominant genotype

2pq= Frequency of heterozygous genotype

Q2 = frequency of homozygous recessive genotype

Example

• If there are two alleles for flower colour (R and r), there are three possible genotypes- RR, Rr and rr. If the frequency of RR (p2) is 0.34 and the frequency of genotype Rr (2pq) is 0.27.

• What is the frequency of the genotype rr?

Tougher Question

• The frequency of cystic fibrosis (genotype ff) in the UK is currently approximately 1 birth in 2000. What proportion of people in the UK are cystic fibrosis carriers (Ff)?

• P2 + 2pq + q2 = 1

Hardy Weinberg Continued..

• The equations can be used to test whether or not the Hardy-Weinberg principle applies to particular alleles in particular populations.

i.e. To test whether selection or any other factors are influencing allele frequencies. If frequencies do change between generations in a large population than there’s a pressure of some kind.