Population genetics deals with: the genetic constitution of populations of interbreeding individuals; factors that cause changes in the constitution over time (generations); and the rates of change. In 1908, Hardy and Weinberg independently realized that the consequence of Mendelian inheritance in a large random-mating population with no selection, mutation, or migration is no change in gene and genotype frequencies over generations (the Hardy-Weinberg equilibrium). Furthermore, one generation of random mating establishes genotype frequencies in Hardy-Weinberg equilibrium. More generally, the relationship between gene and genotype frequencies is determined by the mating system, and hence the latter is an important factor in determining the genetic constitution of populations. Inbreeding results in a decrease in the frequency of heterozygotes and leads to inbreeding depression for traits where dominance acts in the same direction over loci.
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