Question

Brown hair (B) is dominant to blond hair (b). There are 150 brown haired people in a population of 200.

a. What is the frequency of heterozygotes?
b. What is the frequency of homozygous dominant?
c. What is the frequency of the b allele?

Answer 1

a. What is the frequency of heterozygotes? 50%, heterozygotes are the ones that have one of each alelle (B and b). They will have brown hair as a phenotype.

b. What is the frequency of homozygous dominant? 25%, homozygotes are the ones that have the same allele twice (BB). They will also have brown hair.

c. What is the frequency of the b allele? 75%, compound of 50% brown hair and 25% blond hair.

150 represents 75% of 200. To achieve this percentage, two heterozygotes have to be cross:

Bb × Bb

↓ ↓ ↓ ↓

BB Bb bB bb

Because brown hair is dominant, each pair of alleles that has a B will have brown hair as a phenotype.

The pair of alleles that has 2b is the only one that will have blond hair (in this case 25%).