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A Guide to Solving Almost Any HardyWeinberg Genetics Problem
By Ruth Kirkpatrick
For Two Alleles
Step 1:
First write down these two equations.
They are always true.
p + q = 1
where p = frequency of allele A, q = freq. of allele B
AA + AB + BB = 1
where AA = freq. homozygous dominant genotype
AB = freq. heterozygous genotype
BB = freq. homozygous recessive genotype
Step 2:
Does the problem allow you to assume that the alleles are in Hardy Weinberg
equilibrium?
If so, you can write down this third equation:
P
2
+ 2 pq + q
2
where p
2
= frequency of AA
2pq = frequency of AB
q
2
= frequency of BB
If you aren't given this as an assumption, the problem often will be asking you to figure
out if the alleles are in fact, in equilibrium.
Step 3:
What information are you given about allele or genotype frequencies? It is often
possible to figure out all the other alleles and genotype frequencies by the process of
deduction, and you can do this before directly attacking the question at hand.
Are you given the frequency of an allele p?
Find q = 1p.
Next, if you can assume H
W, you can figure out the genotype frequencies using equation 3: p
2
= AA, 2pq = AB, q
2
= BB.
Are you given either of the homozygous genotypes AA or BB?
If you can assume HW,
take the square root of the frequency using p
2
= AA or q
2
= BB and then deduce the allele
frequencies.
If you are just given the heterozygote frequency, then this is one situation
where you cannot deduce the allele frequency when provided with only this information.
Are you given either the recessive or dominant phenotype?
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 Spring '07
 Carlson,Mischel,Power
 Genetics

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