# Need Help: Hardy Weinberg Problem

#### jasminetea7

10+ Year Member
This is one of the problems from Destroyer:

The frequency for the allele for cystic fibrosis is 1 in 2500 (0.0004) among white Americans. What is the heterozygote frequency?

Now, I understand that I should use these two equations:

p+q = 1
p^2 + 2pq + q^2 = 1

I read over the solution and the answer is 4% (2pq = 0.04).
My question is, how do you know which number should be plugged in for which equation? (How do you know that 0.0004 should be plugged in for q^2, not q?)

If you could explain some kind of strategy as well, not for just this particular problem but in general, that would be great. Is there a word or some kind of clue that I should look for when I do these problems? (i.e., allele, heterozygote, etc.)

I get really lost whenever I face these Hardy Weinberg problems....

#### sfoksn

10+ Year Member
well genotypic frequency of homozygous recessive is 1/2500, so to find the allelic frequency of the recessive allele would just be square root of 1/2500.

1- recessive frequency is the dominant allele frequency. Then just do 2*p*q.