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Hardy-Weinberg Equation
The Hardy-Weinberg is an equation used to find the proportion of alleles or phenotypes in a
population
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The final answer may require a percentage but working is always done in decimals
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The first step is to
convert the percentage into a decimal
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For example:
The shells of a snail may be unbanded or banded
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The allele for unbanded, B, is dominant to the allele for banded, b
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Use the Hardy-Weinberg equation to
calculate the percentage of this population that you would expect to be heterozygous for this
gene
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These
are:
BB or Bb

From the equation: p2 = BB, 2pq = Bb
So p2 + 2pq = 0
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51
q2 = 0
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Note: if you are unsure of calculations always use a calculator to be
sure and avoid errors
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7
Now we know the frequency of allele q in the population of snails, so to find p we use the second
equation: p + q = 1
Rearrange: p = 1 – q
p = 1 – 0
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3
We have now found the frequency of allele p in the population of snails
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3 x 0
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42
The final step is to convert the answer back into a percentage
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42 x 100 = 42%

The equation can also be applied to codominance questions
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The allele C B
codes for black feathers and the allele CW codes for white feathers
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As the alleles are codominant, it does not matter which you put as p and which you put as q
CB = p2
CW = q2
CBCW = 2pq
On a farm, 4% of the chickens were black feathered
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First, convert percentage to decimal
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04
Black feathered chickens have the genotype CBCB so p2 = 0
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p = 0
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2
q = 0
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2 x 0
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32
Lastly, convert decimal to percentage:
0
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1
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Brown horses have the genotype BB or Bb
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The frequency of the BB genotype is 0
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What is the frequency of heterozygous horses?
What is the frequency of the B allele?
What is the frequency of the b allele?
2
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64% of monkeys are
long haired

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