aqa a2 Simple question

I know this is a really simple question, but I just don't get it:

Greater willow herb is a common plant found in damp places. It usually has red flowers controlled by the allele R. Plants with the geneotype rr, however, have white flowers. In a sample of plants growing beside a ditch, 17 had white flowers and 327 had red flowers.
Calculate the frequency of the r allele in this sample?

The answer is 0.22 (from the mark scheme) How do you get that? Can anyone tell me the steps to work out one of these questions b/c i started doing 17 / 344 which is totally wrong!

Sorry im sure its a really simple simple question, but they are the hardest ones i find!

Reply 1

Comp_Genius

red flowers genotype = RR or Rr
white flowers genotype = rr
f(RR) = p^2
f(Rr) = 2pq
f(rr) = q^2
thus 17 = q^2
since p^2 + 2pq = 327, p = 14.42
thus f(Rr) = 118.9, and since f(rr) = 17
f(r) = 118.9 + 17 x2 = 153
allele freqeuency = 153/688= 0.22