Hardy Weinberg Equilibrium

The Hardy-Weinberg principle (proposed in 1908 by G. H. Hardy and Wilhelm Weinberg) states that in a large, randomly mating population, the allele frequencies and genotype frequencies stay constant from one generation to the next, provided no evolutionary forces act on the population. A population in this state is in Hardy-Weinberg equilibrium.This may seem surprising: you might expect a dominant allele to become more common over time. It does not. Inheritance itself does not change allele frequencies, because meiosis and fertilisation only reshuffle the alleles already in the gene pool. A recessive allele is not lost; it is carried unchanged in heterozygotes.For a gene with two alleles, the frequencies are given the symbols:• p = frequency of the dominant allele
• q = frequency of the recessive allelep + q = 1Worked example: if the frequency of a recessive allele is q = 0.3, then p = 1 − 0.3 = 0.7. If the population is in equilibrium, these values will still be 0.7 and 0.3 in the next generation, and the one after.The principle is a null model. Real populations are rarely exactly in equilibrium, and that is the point: if the allele frequencies measured in a population change between generations, something is causing evolution, such as natural selection. Comparing real data with Hardy-Weinberg predictions lets biologists detect and measure evolution. The conditions required for equilibrium are covered in a later lesson.