# A level bio help

Hi! Pls can someone help with this question and give a step by step answer

Polydactyl in cats is an inherited condition in which cats have extra toes. The allele for polydactyl is dominant.
In a population 19% of cats have extra toes. Use the Hardy-Weinberg equation to calculate the frequency of the recessive allele for this gene in this population
Original post by rachaeljones
Hi! Pls can someone help with this question and give a step by step answer

Polydactyl in cats is an inherited condition in which cats have extra toes. The allele for polydactyl is dominant.
In a population 19% of cats have extra toes. Use the Hardy-Weinberg equation to calculate the frequency of the recessive allele for this gene in this population

Hardy-Weinberg equations:
p+q=1
p^2 + 2pq + q^2 = 1

So, cats without polydactyl are 0.81, which is pp.
0.19 is Pp and PP, as the polydactyl allele is dominant.
Substituting into Hardy-Weinberg:
p = sqrt(0.81) = 0.9
So as p+q=1, q = 0.1

p is the recessive allele - so its frequency is 0.9 or 90%.