Biology Reference
In-Depth Information
intelligence, selection of an STR through sexual selection is unlikely and has not
been demonstrated.
No migration
Human history is full of migrations and this obviously can lead to changes in the gene
pools of populations. If two distinct populations are living in the same geographical
area and they have different allele frequencies, each population can be in HWE. If
the two different populations are not recognized within the larger population and
are not treated as separate populations, deviation from the HWE may be apparent;
this is known as the Wahlund effect [7 - 9]. If random admixture occurs between the
two populations, the admixed population would be in HWE after one generation.
In reality, where two populations have differences in language, culture or religion,
admixture is normally a much longer process.
Natural selection
At some loci in the human genome the effect of selective pressures can be detected,
for example lactase persistence that is present in populations where milk has been
a sustained part of the diet [10, 11]. Mutations that can confer disease resistance
can also exhibit strong selection effects. The mutation CCR5-
32, which is thought
to offer protection against the haemorrhagic plague that led to vast numbers of
Europeans dying between 1347 and 1670 AD, occurs at a frequency of almost zero in
Asian, African and American Indian populations, whereas it is present at a frequency
of 0.16 (16%) in European populations [12]. However, the loci that are used for
forensic testing are not located within functionally important regions of the genome
and there is no evidence that they are under selective pressure [13].
Mutation
Mutation at STR loci is relatively rapid and it is the instability at these loci that leads
to their high levels of polymorphisms - a trait that makes them valuable genetic
markers. However, the mutation rates of STRs are still relatively low at less than
0.2% per generation and do not have a significant effect on the allelic frequencies
within a gene pool of different or even mixed populations [14 - 18].
Statistical tests to determine deviation from the Hardy-Weinberg
equilibrium
Given that no human population can meet the requirements of the HW law can
we then use it to calculate genotype proportions based on allele frequencies? The
answer from most forensic scientists is yes - because we can empirically measure
the predicted genotype frequencies under HWE and detect if there is a significant
amount of deviation.
