Hardy-Weinberg / Allele Frequency HELP?

[jacobaf20]

I'm having a lot of trouble getting a grasp on this hardy-weinberg thing. In fact, I just did three large problems only to discover I'd done them all incorrectly (I think). Can I get some help?

Q: In a population of 500 individuals, 336 can taste PTC, a dominant trait.
a) What are the allele frequencies?
b) What are the expected numbers of people in the population for each genotype?

a) 500-336=164 recessive, aa. 164/500=33% of the population is recessive.
p+q=1, so x+.33=1, x=.67 or 67% dominant. Is this correct?

b)AA = p^2 = .67^2 = 45% ; .45 x 500 = 225
Aa = 2pq = 2(.67)(.33) = 44% ; .44 x 500 = 220
aa = q^2 = .33^2 = 11% = .11 x 500 = 55

It seems to make sense to me, mathematically, but when I look back on it, I get confused. If 336 people are dominant or have the dominant gene, wouldn't that mean that I would somehow have to get 164 people having homozygous recessive in the second part (b)?

I wonder if I did something wrong in part a--the professor was not very clear about how I get p and q, so some insight into that might help. I've looked online and found some unclear explanations. I think I might actually be getting p^2 and q^, and need to find the square root of the numbers? ...yes?

[phamquocdat41191]

(sorry if my english is unclear)
first of all, can you clarify to me this: 336 can taste PTC. There are two ways to understand what you said: first, it may be all of 336 people are homozygous dominant, second, 336 includes both homozygous dominant and hybrid.
Second, you have to find the square root of 33% to get q. remember hardy weinberg equation: p^2 + 2pq + q^2=1 and p+q=1. 33% here is q^2, not q. find q then you get p. After that, in part b, find p^2, 2pq, q^2.
Third, i tried both ways to find the answer and the answers for both don't make sense to me. They seem not to match to HW equation. Please check that again.
Finally, i'm not sure my explanation is 100% right. Thus get more explanation for this.

[blcr11]

I may be saying the same thing as pharmquocdat41191 is, but I think you are confusing q with q^2. When you determine the number of recessive genotypes, you are finding q^2 = 1/3. Try it this way and I think you'll get the right answer.