If there is no selective pressure imposed on a pair of alleles, one dominant the other recessive, their frequencies should remain constant. This is known as genetic equilibrium.
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The Hardy-Weinberg Law states:
both gene and genotype frequencies will remain unchanged -- in equilibrium -- unless outside forces change those frequencies.
How the Hardy-Weinberg Law works.
When only 2 alleles (A, a) are involved, the relative proportions -- that is the frequencies of A & a -- must equal 1.
If in a population where 80% of the alleles of the gene under study are allele A, the frequencies will be: A= 0.8 and a = 1- 0.8 = 0.2
Mathematically the Hardy Weinberg law is stated as:
A2 + 2Aa + a2 = 1
using the information above we can find the frequencies of the homozygous and heterozygous genotypes;
AA = (.8)2 = .64
2Aa = 2(.8 x .2) =.32
aa = (.2)2 = .04
The modified Punnett square would be:
If we can identify the individuals in a population that are homozygous for a particular allele of interest we can then calculate the frequency of that allele. If for instance 1/10,000 babies are born with a certain recessive genetic defect we can easily determine the allele frequencies of A & a
a = = 0.01
In order for allele frequencies to remain unchanged (i.e. no evolution) the following restrictions must hold:
- Mating must be completely random
- There can be no mutations
- Gene flow must not occur (no migration)
- The population in question must be very large (infinite in size)
- The alleles must segregate according to Mendel's first law
- There can be no selection on the population
Links to Hardy-Weinberg Law