Hardy-Weinberg Equilibrium and Factors Affecting It

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The Hardy-Weinberg Law states that the genotype frequencies of a population will remain in equilibrium, given that the population size is infinitely large and randomly mating. Migration, mutation, selection and genetic drift are also assumed to be absent. Mathematically, if the frequency of an allele (A) is p and the frequency of another allele (a) is q (p+q = 1), the genotypic ratio of AA:Aa:aa in the following generations will remain in the ratio p2:2pq:q2 (p2+2pq+q2 = 1). This ratio is known as the Hardy-Weinberg equilibrium. In reality, it is very unlikely that the above criteria are all met in a population, but the Hardy-Weinberg equilibrium is still useful as it functions as a null hypothesis. If the expected frequencies of the genotypes differ from that predicted from the Hardy-Weinberg equilibrium significantly, we could conclude that some aspects in the assumptions of the equilibrium are not fulfilled. In a wild population, there usually exists natural selection and this is the main driving force of changing allele and genotype frequencies. For natural selection to occur, there must exist differential fitness between genotypes, i.e. individuals with different genotypes have different chances of survival. Also, there must be also a selection force to select for the fitter genotypes. An example in the biological world is peppered moth. Before the industrial revolution in the UK, the majority of peppered moths are white in colour, as this act as a camouflage to protect them from predators as they reside on white trees. However, during the industrial revolution, soot from factories have stained trees grey, favouring the survival of mutant grey peppered moths. In several months, the grey peppered moths gain in population and the white peppered moths decline. Apparently, the colour of trees act as a selection force, altering the genotype frequency of

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