If we want to know the allele frequency of a disease that occurs in 1% of the population, you need to take the following steps:
Know that the percentage of people with the disease is the value for q² in the Hardy-Weinberg equation for genotype frequency:
p² + 2pq + q² = 1
Take the root of q² to determine the allele frequency of the mutated allele, which is 0.1.
Subtract this value from 1, which is the total allele frequency, in order to obtain the value of p. This calculation is based on the Hardy-Weinberg equation for allele frequency:
p + q = 1
You will obtain the following result:
p = 0.9 and q = 0.1
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