Biology Hardy-Weinberg problem A Level

The question says 'in corn, purple kernels are dominant to yellow. A random sample of 100 kernels is taken from a population in Hardy-Weinberg equilibrium. It is found that 9 kernels are yellow and 91 kernels are purple/
What is the frequency of the yellow allele in this population?'
The answer is 0.3, can someone explain how to do this and why what I am doing currently is wrong please? I did it by 91/100 is 0.91 which is purple kernels so p because its dominant and the p+q=1 to 1-0.91 to get 0.09. I'm also not sure whether this is giving for example p or p^2. So if someone could explain when to know the difference of what you are given too please?
Thank you

Reply 1

Super Sprinkles

q is probability (/frequency) of the recessive allele, p is the probability of a dominant allele.

Always look for q^2 first. This is the probability of a recessive phenotype (has two recessive alleles) = 0.09 in this example.
Therefore q = square root of 0.09 = 0.3

For other questions you might need to do some further calculations:
p + q = 1 as you said, so you can find the probability of a dominant allele (p) now that you know q
Probability of a heterozygote = 2pq (you can get dom rec or rec dom)
Probability of a homozygous dominant phenotype = p^2
Probability of a dominant phenotype = 2pq + p^2

Number of [a specific type of] organism = probability of that type of organism * total number of organisms

(edited 2 months ago)

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