Chapter 21, Problem 1QC

Summary Introduction

To determine:

The allele frequency for A and a from the genotype frequencies of aa, Aa and AA.

Concept introduction:

Alleles are the variants of the same gene. Like the gene for height has two alleles T for tall and t for short height. A gene may have more than two alleles as well like in the case of ABO blood grouping. To study the genetic variations, the knowledge of allele frequency is a pre-requisite. It is the number of allele in question divided by the total alleles of the same gene in a population.

Explanation of Solution

The allele frequency for an allele is calculated as the total number of this particular gene in a population divided by the total number of all the alleles for that character in the population.

The allele frequency for A and a from the genotype frequencies of aa, Aa and AA can be derived as follows:

(1) Allele frequency for a:

$\text{Allele Frequency (}a\text{) = }\frac{\text{2×(number of }aa\text{) + 1}\times \text{(number of }Aa\text{)}}{\text{2×(total number of individuals)}}$

We multiply two to different numbers because each individual is diploid and has two alleles. However, in case of Aa it has only one a thus we do not multiply it with two.

(2) Allele frequency for A:

$\text{Allele Frequency (}A\text{) = }\frac{\text{2×(number of }AA\text{) + 1}\times \text{(number of }Aa\text{)}}{\text{2×(total number of individuals)}}$

The same reasons for multiplication with two apply here as they were for the allele frequency of a.

Moreover, if the frequencies are converted to percentage by multiplying with hundred, then the subtraction of allele frequency of a from hundred will directly give the allele frequency for A and vice versa.

Conclusion

Allele frequency for all the alleles of a gene must be known when studying the genetic variations of a trait expressed by the gene in question.