Deviation from the Hardy-Weinberg equilibrium | Kickoff

Deviation from the Hardy-Weinberg equilibrium

12 Apr

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;
no mutations occur.

Clearly no human population will meet these criteria and they will deviate from HWE 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 [3]. However, most populations are sufficiently large for allele frequencies not to be significantly affected by genetic drift. 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 [4, 5].

STATISTICAL TESTS TO DETERMINE DEVIATION FROM THE HARDY-WEINBERG EQUILIBRIUM

Random mating
Humans clearly do not mate completely randomly. However, because STR genotypes do not have any impact on a person’s phenotype, such as height, strength or 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, there can appear to be deviation from the HWE; this is known as the Wahlund effect [6-8]. 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 [9, 10]. Mutations that can confer disease resistance can also exhibit strong selection effects. The mutation CCR5- 32 allele, that is thought to offer protection against the haemorrhagic plague that led to vast numbers of Europeans dying between 1347 to 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 [11]. 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.

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 [12-15].

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

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