What do I do with the Hardy-Weinberg equation?

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p2+ 2pq q2= 1 


It looks daunting at first but once you get the hang of them they can be easy marks in the exam. p is the frequency of the dominant allele in the population, and q is the frequency of the recessive allele in the population. p2 is two dominant alleles and so represents the frequency of the homozygous dominant genotype (pp), and q2 is two recessive alleles and represents the frequency of the homozygous recessive genotype (qq). 2pq represents the frequency of the heterozygous genotype (pq). By substituting different numbers into the frequencies you can work out the frequency of a genotype in a population, there are of course limits to the accuracy of this, but it is still a very useful equation. You do not need to know how the equation came to look how it does but in case you are interested or it helps you learn I have outlined it below:

For a population in genetic equilibrium:

p+q=1 (the sum of both allele frequencies is 100%)

(p + q)2 =1


p2 + 2pq q2 = 1 

In a tutoring session I would walk you through multiple examples and give you past paper equations to try, I have a few tips and tricks about this topic which helped me during my exams.

Atalie S. A Level Biology tutor, GCSE Biology tutor

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