Question

How do you use the Hardy-Weinberg principle?

Answer 1

Hardy Weinberg principle is used to derive frequencies of different alleles of a particular gene, within a subpopulation and it also helps to determine genetic load.

Explanation:

Hardy Weinberg principle is variously known as HW law/theorem. Population geneticists term it as HW equilibrium. According to this, the frequencies of different alleles of a gene will remain constant in a population, generation after generation, in absence of active evolutionary forces.

If we consider a gene with two alleles B, for dominant allele and b for recessive allele, then frequency of allele B is represented by p and that of allele b is represented by q. Obviously p plus q would always add upto 1.

According to HW law:

`(p+q)^2`= `p^2 + 2pq + q^2` = 1

`p^2` is the frequency of homozygous dominant individuals BB
`2pq` is the frequency of heterozygous dominant individuals Bb
`q^2` is the frequency of homozygous recessive individuals bb

The idea inlaid in HW law is used in demographic studies, to determine allele frequencies. In a subpopulation(=deme), recessive individuals (bb) are easily identified due to phenotypic difference from dominant individuals (BB, Bb), hence `q^2` is known. Thus frequency of recessive allele ( q ) could be determined. As we know that p+q=1, and as q is known, frequency of dominant allele ( p) could also be determined.

Let us consider the example of albinos in a small population where there is one albino in every hundred individuals (`q^2` = 1/100 = 0.01), which means frequency of recessive allele is 0.1 (`q` = 0.1). That means frequency of dominant allele for normal skin color is 0.9 (as p+q = 1). [Please note this is a hypothetical proposition, could be possible in isolated population: in the world 1 in 20,000 are albino].

Now you see 2pq or 2x0.9x0.1 i.e. 18/100 is the number of heterozygous dominant individuals in this particular population who all are carriers of the allele responsible for causing albinism, though you see only one individual affected with the condition.

Thus HW law also helps you to find out 'genetic load' i.e. how much of an undesirable allele could be present in the gene pool.