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# Hardy-Weinberg principle

The Hardy-Weinberg principle (HWP) (also Hardy-Weinberg equilibrium or HWE) states that after one generation of random mating[?], the genotype frequencies at a single gene (or locus) can be represented as a simple function of allele frequencies at that locus. In the simplest case of two alleles A and a with allele frequencies of p and q, respectively, the HWP predicts that the genotypic frequencies for the AA homozygote to be p2, the Aa heterozygote to be 2pq and the other aa homozygote to be q2.

The Hardy-Weinberg principle is the consequence of Mendel's First Law (the law of independent segregation) at the population-level and is a basic principle of population genetics. First formulated independently in 1908 by English mathematician G. H. Hardy and German physician Wilhelm Weinberg[?] the original assumptions for Hardy-Weinberg Equilibrium (HWE) were that populations are:

and experience:

#### Derivation of the Hardy-Weinberg principle

A more statistical description for the HWP, is that the alleles for the next generation for any given individual are chosen independently. Consider two alleles, A and a, with frequencies p and q, respectively, in the population then the different ways to form new genotypes can be derived using a Punnett square[?], where the size of each cell is proportional to the fraction of each genotypes in the next generation:

Females A (p) AA (p2) Aa (pq) aA (qp) aa (q2)

So the final three possible genotype frequencies, in the offspring, if the alleles are drawn independently become:

• p2 (AA)
• 2pq (Aa)
• q2 (aa)

The generalization of the HWP for more than two alleles, can be found by the multinomial formula. If the frequencies of the n alleles at a given locus A1,...,An are given by p1,...,pn, then the frequency of the AiAj genotype is given by:

• 2pipj if ij and;
• pi2 if i=j.

Testing deviation from the HWP is generally performed using Pearson's chi-square test, using the observed genotype frequencies obtained from the data and the expected genotype frequencies obtained using the HWP. For systems where there are large numbers of alleles, this may result in data with sparse cells, because there are often not enough individuals present in the sample to adequately represent all genotype classes. If this is the case, it may be necessary to use a form of Fisher's "exact test".

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