Genetics Question

[king321]

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Hey guys I don't understand this question in bootcamp. This questions seems pretty simple but has a complicated answer. I thought the answer should be .91 but the answer is .70.
A student observes a population of 100 eagles in Hardy-Weinberg equilibrium. The eagle’s allele for a pointed beak is dominant, and the allele for a rounded beak is homozygous recessive. If 9 eagles were found to have a rounded beak in the population, what is the allele frequency of the pointed beak?
A. .09
B .30
C .70
D .81
E .91

 

[dawg289]

Your answer would be correct if the question was "what is the frequency of the dominant phenotype?". However, there are two possible combinations that can lead to the dominant phenotype: homozygous dominant and heterozygous.

Let A = dominant allele
and a = recessive allele
If 9 eagles were found to have a rounded beak.
.09 = a^2
a=.3
A=1-a=.7

Note how dominant phenotype = A^2+2Aa = .91