In a population of 10,000 individuals, 400 express a recessive trait. How many heterozygotes are there expected to be?

We will use the Hardy-Weinberg equation; p2 + 2pq + q2 = 1

q2 represent the frequency of the recessive trait in the population which is 400/10000 = 0.04. Then find frequency of the recessive allele.

q = 0.2. While we understand that p+q = 1; p = 1-q; p = 1-0.2 = 0.8.

2pq represents the frequency of heterozygous trait in the population. Therefore 2*0.8*0.2 = 0.32

0.32*10000 = 3200