When Is Heterozygosity Highest? Understanding Allele Frequencies and Heterozygote Proportions

In population genetics, understanding how allele frequencies influence the proportion of heterozygotes is fundamental. The Hardy-Weinberg equilibrium provides a mathematical framework to predict genotype frequencies, including the frequency of heterozygotes, in a population. But when is this frequency at its maximum?

The Hardy-Weinberg Equation and Heterozygosity

For a gene with two alleles, A and a, the Hardy-Weinberg equation is:

p2+2pq+q2=1p2+2pq+q2=1

• $p$ = frequency of allele A

• $q$ = frequency of allele a

• $2pq$ = frequency of heterozygotes (Aa)

Maximizing Heterozygosity

The frequency of heterozygotes ($2pq$) depends on the values of $p$ and $q$. Since $p+q=1$, heterozygosity is maximized when both alleles are present in equal frequencies:

• If $p=0.5$ and $q=0.5$:

$$2pq=2\times 0.5\times 0.5=0.5$$

This is the maximum possible frequency of heterozygotes in a population under Hardy-Weinberg equilibrium.

• If either allele is more common than the other (for example, $p=0.8,q=0.2$), the value of $2pq$ decreases:

$$2pq=2\times 0.8\times 0.2=0.32$$

Why Equal Frequencies Matter

When both alleles are equally frequent, the chance of randomly pairing different alleles is highest, leading to the greatest proportion of heterozygotes. As the frequency of one allele increases and the other decreases, the likelihood of forming heterozygotes drops.

Conclusion: The Correct Condition

The frequency of heterozygotes will be highest when the frequency of ‘A’ is equal to the frequency of ‘a’.

Correct answer: (3) Frequency of ‘A’ is equal to frequency of ‘a’