What is the frequency of individuals expressing the dominant trait?

In a certain genetically stable population, the frequency of a recessive allele (for a trait with two alleles) is 0.6.

What is the frequency of individuals expressing the dominant trait?
A . 0.64
B . 0.36
C . 0.24
D . 0.16

View Answer

Answer: A

Explanation:

The question stem asks you to determine the frequency of individuals expressing the dominant trait in a genetically stable population. However, before you do that, you need to determine the allelic frequencies in the population. This question involves a practical application of the Hardy-Weinberg equation. The Hardy-Weinberg equilibrium states that within a genetically stable population, the gene frequencies of dominant and recessive alleles will not change over time. Two mathematical expressions are associated with the Hardy-Weinberg equilibrium. The first relationship, p + q = 1, describes the relative allelic frequencies in a population. p is defined as the frequency of the dominant allele and q is defined as the frequency of the recessive allele, and the sum of both those frequencies adds up to 1, or 100%. The second relationship, p2 + 2pq+ q2 = 1, describes the relative genotypic frequencies in the population. p2 represents homozygous, or dominant pp genotypes; q2 represents homozygous, or frequency of the dominant allele, p, by the mathematical relationship p + q = 1. Therefore, the frequency of p is 0.4 because 0.6 + 0.4 = 1. Next, you need to determine the frequency of individuals expressing the dominant trait by recessive qq genotypes; and 2pq represents the frequency of heterozygotes, or hybrids.applying the second relationship, p2 + 2pq+ q2 = 1. The individuals expressing the dominant trait are those that have the pp and pq genotypes, so to find the total frequency of individuals expressing the dominant trait, you add p2 and 2pq. Thus, p2 = 0.4 × 0.4, or 0.16 and 2pq = 2 × 0.6 × 0.4, or 0.48. If you add the two together, you get 0.16 + 0.48, or 0.64. Thus, 0.64 is the correct frequency of individuals expressing the dominant trait.