Biology Reference
In-Depth Information
A (
p
= 0.6)
B (
q
= 0.4)
AA (
p
2
= 0.6
2
= 0.36)
A (
p
= 0.6)
AB (
pq
= 0.6 x 0.4 = 0.24)
BB (
q
2
= 0.4
2
=
0.16)
B (
q
= 0.4)
AB (
pq
= 0.6 x 0.4 = 0.24)
Figure 8.1
A Punnet square showing the relationship between alleles A and B, along with all the
possible resulting genotypes. If allele A occurs at a frequency (
p
) of 0.6 and allele B occurs at
frequency (
q
) of 0.4 then it is possible to estimate that the population will contain individuals
with genotypes AA, AB and BB at frequencies of 0.36, 0.48 and 0.16 respectively. Each homozygote
appears only once, hence
p
2
,or
q
2
. The heterozygote will be represented twice, hence 2
pq
heterozygotes can be calculated using 2
pq
, removing the need to construct elaborate
Punnet squares.
Deviation from the Hardy-Weinberg equilibrium
The HW law states that certain conditions must be met. These are:
•
the population is infinitely large;
•
random mating occurs within the population;
•
the population is free from the effects of migration;
•
there is no natural selection; and
•
no mutations occur.
Clearly no human population will meet these criteria and all will deviate from the
HW equilibrium to a greater or lesser extent.
Infinitely large population
A consequence of finite population size is that the frequency of alleles will change
through a process known as random genetic drift, where the frequency of any given
allele will increase or decrease through chance events. The effect of genetic drift is
more pronounced in smaller populations [4]. However, most populations are suffi-
ciently large for allele frequencies not to be significantly affected. Even in relatively
small isolated human populations, it has been shown that alleles that are present at
a frequency of more than 1% are rarely lost in recently diverged populations [5, 6].
Random mating
Humans clearly do not mate completely randomly. However, because STR geno-
types do not have any impact on a person's phenotype, such as height, strength or
