Question

You take genotypes from 500 people for a particular gene with two alleles, and find the following: homozygous dominant: 401 people homozygous recessive: 8 people heterozygous: 91 people a) What is the frequency of the dominant allele? b) What is the frequency of the recessive allele?

Answer 1

Answer: When we talk about Genotypes, we are aware that Genotype is a particular combination of alleles for a particular gene. For an example in cystic fibrosis, people with the normal-mutant or normal-normal genotypes are found to have the normal phenotype, while people with the mutant-mutant genotype are found to have the disease phenotype

Homozygous dominant- Homozygous means that individual has two copies of a particular allele for a gene. In Homozygous dominant individual carries the two copies of same dominant allele. That means individual will show the dominant phenotype.

Homozygous recessive- In this case individual carries two copies of same recessive allele. That means the individual will show the recessive phenotype.

Heterozygous- In this case individual has two different alleles of a gene. That means individual will show the dominant phenotype as in this case individual might have one of the dominant allele of a gene.

The find out the frequencies of dominant and recessive alleles we must apply Hardy-Weinberg principle; this principle is used to compare the frequencies of allele in a given population over a period of time.

So to apply this rule a population of alleles are considered to be in an equilibrium and for that these allele must follow these five rules-

1) No gene mutations may occur and therefore allele changes do not occur.

2) There must be no migration of individuals either into or out of the population.

3) Random mating must occur, meaning individuals mate by chance.

4) No genetic drift, a chance change in allele frequency, may occur. 5) No natural selection, a change in allele frequency due to environment, may occur.

But this equilibrium is never occurs in nature as one of the rule is always been changed. It is said to be the idea state against which scientists measures the gene evolution of a population.

The two equation involved in Hardy-Weinberg principle are-

? + ? = 1

?² + 2?? + ?² = 1

Where,

               ? - The frequency of the recessive allele

             ?²-The frequency of individuals with the homozygous dominant genotype

             ??²- The frequency of individuals with the homozygous recessive genotype

            ? - The frequency of the dominant allele

            2??- The frequency of individuals with the heterozygous genotype

With reference to the question,

Given,

            Homozygous dominant      :      401 people

            Homozygous recessive       :      8 people

            Heterozygous                    :      91 people

As we know that homozygous dominant and heterozygous both will show the dominant phenotype so we will add up the values given to have the dominant phenotype giving population

Homozygous dominant + Heterozygous = 492

i.e. Dominant phenotype are in total 492

      Recessive phenotypes are Homozygous recessive = 8

      Total population = 500

To put all these given values in a Hardy-Weinberg principle equation we will have to find out the values for each variable been used that is ?, ?, ?², 2??, and ?²

First step: Find the frequency of homozygous recessive genotype -

   It can be determined by,

Frequency of individuals =