Use the following information to answer the questions below.

A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants.

What proportion of the population is probably heterozygous (Aa) for this trait?

Select one:

a. 0.05

b. 0.25

c. 0.50

d. 0.75

e. 1.00

Answer: c - The proportion of the population that is heterozygous (Aa) for this trait is 0.50

Explanation: Percentage of those with homozygous recessive alleles (aa) in the population has been given as 25%. We can represent this as q².

Recall that Hardy-Weinberg provides the equation p² + 2pq + q² = 1, where

P² = homozygous dominant alleles

q² = homozygous recessive alleles

2pq = heterozygous alleles

q² = 25% = 25/100 = 0.25

q = √0.25 = 0.5

Also recall that for two alleles, p and q, p + q = 1.

Therefore, p = 1 - q = 1 - 0.5 0.5

Since we now have the values of p and q, we can calculate the proportion of heterozygous alleles (Aa) as 2pq = 2 x 0.5 x 0.5 = 0.5