Hardy–Weinberg law
The Hardy–Weinberg law was developed independently by an English mathematician, G.H. Hardy, and a German doctor, Wilhelm Weinberg.[1][2] This concept is also known as the Hardy–Weinberg equilibrium, Hardy–Weinberg theorem or Hardy–Weinberg principle. Sometimes Weinberg's name is placed first.
The law is a foundation of population genetics, and it is still taught to students today.[3] It states that the proportions of alleles of all genes in any population will remain the same unless perturbed (disturbed). That applies to all loci on all chromosomes in the population. Possible perturbations are:
- ♦ gene mutation
- ♦ natural selection
- ♦ small population size where random effects like genetic drift and inbreeding may occur. H/W populations are assumed to be infinite in size.
- ♦ assortative mating instead of random mating. In effect, this would split the population down into small groups, see item above.
- ♦ migration into or out of the population under study.
It follows that any systematic change in the frequency of alleles in a population must be due to the effect of one or more of these causes. Of course, like all aspects of Mendelian inheritance, the expected proportions of alleles are probabilities. It was for that reason that statistical tests of significance, such as standard errors, were developed.
Although all changes must be due to perturbations, not all perturbations lead to changes. The classic case is balancing selection, such as heterozygote advantage: "Heterosis: the heterozygote at a locus is fitter than either homozygote".[4][5][6] Balancing selection leads to an equilibrium population with Hardy–Weinberg proportions
Generalisation
The law permits the prediction of genotype frequencies from a knowledge of gene frequencies.
- If alleles A and a are in the proportions p and q, the three zygotic types AA, Aa, and aa are in the proportions p2: 2pq: q2.
Thus, equations can be written in terms of allele frequencies, and hypotheses about how phenotypes are inherited can be tested from population data.[7]
Hardy–Weinberg Law Media
Hardy–Weinberg proportions for two alleles: the horizontal axis shows the two allele frequencies p and q and the vertical axis shows the expected genotype frequencies. Each line shows one of the three possible genotypes.
Length of p, q corresponds to allele frequencies (here p = 0.6, q = 0.4). Then area of rectangle represents genotype frequencies (thus AA : Aa : aa = 0.36 : 0.48 : 0.16).
Punnett square for three-allele case (left) and four-allele case (right). White areas are homozygotes. Colored areas are heterozygotes.
A de Finetti diagram representing a distribution of genotype frequencies
References
- ↑ Hardy G.H. (1908). "Mendelian proportions in a mixed population". Science. 28 (706): 49–50. Bibcode:1908Sci....28...49H. doi:10.1126/science.28.706.49. ISSN 0036-8075. PMID 17779291.
- ↑ Weinberg W. 1908. Über den Nachweis der Vererbung beim Menschen. Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg 64: 368–382.
- ↑ Gillespe J.H. 2004. Population genetics: a concise guide. Johns Hopkins, Baltimore MD.
- ↑ Ford E.B. 1975. Ecological genetics, 4th ed. London: Chapman & Hall.
- ↑ Maynard Smith J. 1998. Evolutionary genetics. Oxford. p65
- ↑ Ford E.B. 1965. Genetic polymorphism, p26, Heterozygous advantage. MIT Press 1965.
- ↑ Crow J.F. 1988. Eighty years ago: the beginnings of population genetics. Genetics 119: 473–476