1
Introduction
Modern biologists define evolution as the
change in
allele
frequency in a population
over time. There are several mechanisms
that can cause
a population’s
allele
frequencies to change including
migration, mutation, selection and genetic
drift. If none of these mechanisms are
operating, then allele frequencies will
remain constant from one generation to
the next (i.e. the allele frequencies are in
equilibrium). This central evolutionary
tenet is referred to as the Hardy
Weinberg Equilibrium Theorem.
HardyWeinberg Equilibrium
theory can be summarized with a simple equation that allows us to track both allele and genotype
frequencies in a one gene, two allele model:
1 =
p
2
+ 2
pq
+
q
2
where
p
= frequency of the first allele,
q
= frequency
of the second allele,
p
2
= frequency of the
p
homozygote genotype,
q
2
= frequency of the
q
homozygote genotype and 2
pq
= frequency of the
heterozygote genotype.
For example, suppose a gene has two alleles,
A
and
a.
By sampling the population,
we find the
frequency of the alleles to be A
=
0.2 and a = 0.8
(remember,
p + q
= 1). We can then predict the
genotypic frequencies by assigning
A
and a to the
variables
p
and
q
in the above equation, such that
p
2
=
(0.2)
2
= 0.04,
q
2
= (0.8)
2
= 0.64 and
2pq
= 2 (0.2 * 0.8)
= 0.32 (see Figure 1). Thus, if 20% of the alleles in a
population are
A
, then we expect only 4% of the population to be
AA
homozygous. After our
calculations, if we find the AA genotype to differ from 4% (e.g. 15%), then something has occurred to
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 Fall '11
 Klowden/Crampton
 Genetics, Evolution, Population Genetics

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