Question

A population is in Hardy- weinberg equilibrium for a gene with only two alleles. If the gene frequency of an allele A is $0.7$, the genotype frequency of $Aa$ is

• A. 0.21
• B. 0.42
• C. 0.36
• D. 0.7

Answer 1

Answer: (B)

For a gene with two alleles, $A$ (dominant) and $a$ (recessive), if the frequency of $A$ is $p$ and the frequency of $a$ is $q$, then the frequencies of the three possible genotypes ( $AA,Aa$, and $aa$ ) can be expressed by the Hardy-Weinberg equation:
$p^2+2pq+q^2=1$
where, $p^2=$ frequency of $AA$ (homozygous dominant) individuals, $2pq=$ frequency of $Aa$ (heterozygous) individuals and $q^2=$ frequency of $aa$ (homozygous recessive) individuals. The equation can be used to calculate allele frequencies if the numbers of homozygous recessive individuals in the population is known.
Here, $p=0.7$ and $q=0.3$ (given)
$\therefore 2pq$ (frequency of heterozygote)
$=2\times 0.7\times 0.3=0.42$