# Hardy-Weinberg Law

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## Hardy-Weinberg law

[¦här·dē ′wīn‚bərg ‚lȯ]
(genetics)
The concept that frequencies of both genes and genotypes will remain constant from generation to generation in an idealized population where mating is random and evolutionary forces (such as mutation, migration, selection, or genetic drift) are absent.

## Hardy-Weinberg Law

a law of population genetics that establishes the relationship between the frequencies of the genes and genotypes in a freely interbreeding population. The law was independently formulated in 1908 by the British mathematician G. Hardy and the German physician W. Weinberg.

According to this law, if the number of diploid organisms in a population is so large that one may ignore occasional fluctuations in gene frequencies (that is, genetic drift), and in the absence of mutation, migration, and selection (with respect to the gene in question), the frequencies of genotypes AA, Aa, and aa in a given population, after the first generation, will remain the same in the succeeding generations. According to the Hardy-Weinberg formula, the frequencies of genotypes AA, Aa, and aa will be equal to p2, 2pq, and q2, respectively, where A and a represent the alleles of a gene that is not sex-linked, p represents the frequency of allele A, and q represents the frequency of allele a. The Hardy-Weinberg law also applies to multiallelic genes.

In polyploid populations—as well as in the case of sex-linked genes in diploid populations—the relative proportions are only established over many generations. Even though a given population may conform to the Hardy-Weinberg formula, this is not necessarily proof that the population is unaffected by genetic processes. For example, the interbreeding of closely related individuals (that is, inbreeding), which increases the proportion of homozygotes in the population, when combined with selection against homozygotes may lead to genotype frequencies corresponding to the Hardy-Weinberg formula. By comparing the actually observed frequencies of genotypes to the frequencies theoretically expected according to the Hardy-Weinberg law, it is possible in a number of cases to gauge the frequencies of the alleles, to single out the factors influencing the alleles, and to obtain quantitative data pertaining, for example, to selection, non-random interbreeding, migration, and chance fluctuations.

The concept of the genetic equilibrium of populations, as first expressed in the Hardy-Weinberg law, underlies the modern view of the interacting processes of population genetics.

### REFERENCES

Timofeev-Resovskii, N. V., A. V. Iablokov, and N. V. Glotov. Ocherk ucheniia opopuliatsii. Moscow, 1973.
Mettler, L., and T. Gregg. Genetika populiatsii i evoliutsiia. Moscow, 1972. (Translated from English.)
Li, C. C. First Course in Population Genetics. California, 1975.

L. A. ZHIVOTOVSKII

References in periodicals archive ?
The chapters on genetical basis of plant breeding and quantitative inheritance provide the basics of Hardy-Weinberg law, components of variation, and an extensive treatment of diallel analysis.
The two examples that come readily to mind are the Hardy-Weinberg law that applies to ratios of genes in a population under particular conditions and the law of independent assortment of chromosomes applied to the partitioning of chromosomes during meiosis.
Additional information would have been appreciated, although lengthening the book (for instance, in my lecturing, I discuss the Hardy-Weinberg Law as a powerful null hypothesis for evolution).
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