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The Hardy–Weinberg principle (also known as the Hardy–Weinberg "Equilibrium", "Model", "Theorem", or "Law") states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include non-random mating, mutation, selection, genetic drift, gene flow and meiotic drive. Because one or more of these influences are typically present in real populations, the Hardy–Weinberg principle describes an ideal condition against which the effects of these influences can be analyzed.
In the simplest case of a single locus with two alleles denoted A and B with frequencies freq(A) = p and freq(B) = q, respectively, the expected genotype frequencies are freq(AA) = p2 for the AA homozygotes, freq(BB) = q2 for the BB homozygotes, and freq(AB) = 2pq for the heterozygotes. The genotype proportions p2, 2pq, and q2 are called the Hardy-Weinberg proportions. [Note that (p + q) = (p + q)2 = p2 + 2pq + q2 = 1].
If union of gametes to produce the next generation is random, in can be shown [see derivation] that the new freq(B') = q2 + pq = (q)(q + p) = q. That is, allele frequencies are constant between generations.
Consider a population of monoecious diploids [each organism produces male and female gametes at equal frequency, and has two alleles at each gene locus]. Organisms reproduce by random union of gametes (the “gene pool” population model). A locus in this population has two alleles, A and B. that occur at frequencies p and q, respectively. The different ways to form new genotypes can be shown in a Punnett square, where the proportion of each is equal to the product of the row and column allele frequencies.
|A (p)||B (q)|
|Males||A (p)||AA (p2)||AB (pq)|
|B (q)||AB (qp)||BB (q2)|
The sum of the entries is p2 + 2pq + q2 = 1, as the genotype frequencies for an infinite population size) must sum to one.
Note again that as (p + q) = 1, the binomial expansion of (p + q)2 = p2 + 2pq + q2 = 1 gives the same relationships.
Summing the elements of the Punnet square or the binomial expansion, the expected genotype proportions among the offspring become:
f(AA) = p2
f(AB) = pq + qp = 2pq
f(BB) = q2
These frequencies are called the Hardy–Weinberg proportions.
In the following generation, the frequency of either allele is determined by its expected contribution from the homozygote and heterozygote genotypes, which are 1 and 0.5, respectively, for either allele. Then, the frequency of the parental genotypes weighted by the expected contribution is
p’ = (1)p2 + (0.5)2pq = p2 + pq = (p)(p + q) = p
q’ = (1)q2 + (0.5)2pq = q2 + pq = (q)(q + p) = q
that is, p’ = p and q = q’ and allele frequencies remain constant after one generation of random union of gametes in an infinite population.
For the more general case of dioecious diploids [organisms are either male or female] that reproduce by random mating of individuals, it is necessary to calculate the frequency of the nine possible matings between each genotype (AA, AB, and BB) in either sex, weighted by the expected genotype contributions of each such mating. Then it can be shown:
f’(AA) = p4 +p3q + p3q + p2q2 = (p2)(p2 + 2pq + q2) = p2
f’(AB) = p3q + p2q2 + p3q + 2p2q2 +p2q2 + pq3 = 2p3q + 4p2q2 + 2pq3 = (2pq)(p2 + 2pq + q2) = 2pq
f’(BB) = p2q2 +pq3 + pq3 +q4 = (q2)(p2 + 2pq + q2) = q2
from which allele frequencies may be calculated as above. As before, allele frequencies remain constant after one generation of random mating between separate sexes.
If in either monoecious or dioecious organisms, either the allele or genotype proportions are initially unequal in either sex, it can be shown that constant proportions are obtained after one generation of random mating. If dioecious organisms are heterogametic and the gene locus is located on the X chromosome, it can be shown that if the allele frequencies are initially unequal in the two sexes [e.g., XX females and XY males, as in humans] , f’(B) in the heterogametic sex ‘chases’ f(B) in the homogametic sex of the previous generation, until an equilibrium is reached at the weighted average of the two initial frequencies.
Deviations from Hardy–Weinberg equilibrium 
The seven assumptions underlying Hardy–Weinberg equilibrium are as follows:
- organisms are diploid
- only sexual reproduction occurs
- generations are non overlapping
- mating is random
- population size is infinitely large
- allele frequencies are equal in the sexes
- there is no migration, mutation or selection
Violations of the Hardy–Weinberg assumptions can cause deviations from expectation. How this affects the population depends on the assumptions that are violated.
- Random mating. The HWP states the population will have the given genotypic frequencies (called Hardy–Weinberg proportions) after a single generation of random mating within the population. When the random mating assumption is violated, the population will not have Hardy–Weinberg proportions. A common cause of non-random mating is inbreeding, which causes an increase in homozygosity for all genes.
If a population violates one of the following four assumptions, the population may continue to have Hardy–Weinberg proportions each generation, but the allele frequencies will change over time.
- Selection, in general, causes allele frequencies to change, often quite rapidly. While directional selection eventually leads to the loss of all alleles except the favored one, some forms of selection, such as balancing selection, lead to equilibrium without loss of alleles.
- Mutation will have a very subtle effect on allele frequencies. Mutation rates are of the order 10−4 to 10−8, and the change in allele frequency will be, at most, the same order. Recurrent mutation will maintain alleles in the population, even if there is strong selection against them.
- Migration genetically links two or more populations together. In general, allele frequencies will become more homogeneous among the populations. Some models for migration inherently include nonrandom mating (Wahlund effect, for example). For those models, the Hardy–Weinberg proportions will normally not be valid.
- Small population size can cause a random change in allele frequencies. This is due to a sampling effect, and is called genetic drift. Sampling effects are most important when the allele is present in a small number of copies.
Where the A gene is sex linked, the heterogametic sex (e.g., mammalian males; avian females) have only one copy of the gene (and are termed hemizygous), while the homogametic sex (e.g., human females) have two copies. The genotype frequencies at equilibrium are p and q for the heterogametic sex but p2, 2pq and q2 for the homogametic sex.
For example, in humans red–green colorblindness is an X-linked recessive trait. In western European males, the trait affects about 1 in 12, (q = 0.083) whereas it affects about 1 in 200 females (0.005, compared to q2 = 0.007), very close to Hardy–Weinberg proportions.
If a population is brought together with males and females with a different allele frequency in each subpopulation (males or females), the allele frequency of the male population in the next generation will follow that of the female population because each son receives its X chromosome from its mother. The population converges on equilibrium very quickly.
The simple derivation above can be generalized for more than two alleles and polyploidy.
Generalization for more than two alleles 
Consider an extra allele frequency, r. The two-allele case is the binomial expansion of (p + q)2, and thus the three-allele case is the trinomial expansion of (p + q+ r)2.
More generally, consider the alleles A1, ..., Ai given by the allele frequencies p1 to pi;
giving for all homozygotes:
and for all heterozygotes:
Generalization for polyploidy 
The Hardy–Weinberg principle may also be generalized to polyploid systems, that is, for organisms that have more than two copies of each chromosome. Consider again only two alleles. The diploid case is the binomial expansion of:
and therefore the polyploid case is the polynomial expansion of:
where c is the ploidy, for example with tetraploid (c = 4):
Depending on whether the organism is a 'true' tetraploid or an amphidiploid will determine how long it will take for the population to reach Hardy–Weinberg equilibrium.
Complete generalization 
For distinct alleles in -ploids, the genotype frequencies in the Hardy–Weinberg equilibrium are given by individual terms in the multinomial expansion of :
The Hardy–Weinberg principle may be applied in two ways, either a population is assumed to be in Hardy–Weinberg proportions, in which the genotype frequencies can be calculated, or if the genotype frequencies of all three genotypes are known, they can be tested for deviations that are statistically significant.
Application to cases of complete dominance 
Suppose that the phenotypes of AA and Aa are indistinguishable, i.e., there is complete dominance. Assuming that the Hardy–Weinberg principle applies to the population, then can still be calculated from f(aa):
and can be calculated from . And thus an estimate of f(AA) and f(Aa) derived from and respectively. Note however, such a population cannot be tested for equilibrium using the significance tests below because it is assumed a priori.
Significance tests for deviation 
Testing deviation from the HWP is generally performed using Pearson's chi-squared 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 many empty possible genotypes and low genotype counts, because there are often not enough individuals present in the sample to adequately represent all genotype classes. If this is the case, then the asymptotic assumption of the chi-squared distribution, will no longer hold, and it may be necessary to use a form of Fisher's exact test, which requires a computer to solve. More recently a number of MCMC methods of testing for deviations from HWP have been proposed (Guo & Thompson, 1992; Wigginton et al. 2005)
Example χ2 test for deviation 
This data is from E.B. Ford (1971) on the Scarlet tiger moth, for which the phenotypes of a sample of the population were recorded. Genotype-phenotype distinction is assumed to be negligibly small. The null hypothesis is that the population is in Hardy–Weinberg proportions, and the alternative hypothesis is that the population is not in Hardy–Weinberg proportions.
|Phenotype||White-spotted (AA)||Intermediate (Aa)||Little spotting (aa)||Total|
From which allele frequencies can be calculated:
So the Hardy–Weinberg expectation is:
Pearson's chi-squared test states:
There is 1 degree of freedom (degrees of freedom for test for Hardy–Weinberg proportions are # genotypes − # alleles). The 5% significance level for 1 degree of freedom is 3.84, and since the χ2 value is less than this, the null hypothesis that the population is in Hardy–Weinberg frequencies is not rejected.
Fisher's exact test (probability test) 
Fisher's exact test can be applied to testing for Hardy–Weinberg proportions. Since the test is conditional on the allele frequencies, p and q, the problem can be viewed as testing for the proper number of heterozygotes. In this way, the hypothesis of Hardy–Weinberg proportions is rejected if the number of heterozygotes is too large or too small. The conditional probabilities for the heterozygote, given the allele frequencies are given in Emigh (1980) as
where n11, n12, n22 are the observed numbers of the three genotypes, AA, Aa, and aa, respectively, and n1 is the number of A alleles, where .
An example Using one of the examples from Emigh (1980), we can consider the case where n = 100, and p = 0.34. The possible observed heterozygotes and their exact significance level is given in Table 4.
|Number of heterozygotes||Significance level|
Using this table, one must look up the significance level of the test based on the observed number of heterozygotes. For example, if one observed 20 heterozygotes, the significance level for the test is 0.007. As is typical for Fisher's exact test for small samples, the gradation of significance levels is quite coarse.
However, a table like this has to be created for every experiment, since the tables are dependent on both n and p.
Inbreeding coefficient 
The inbreeding coefficient, F (see also F-statistics), is one minus the observed frequency of heterozygotes over that expected from Hardy–Weinberg equilibrium.
where the expected value from Hardy–Weinberg equilibrium is given by
For example, for Ford's data above;
For two alleles, the chi-squared goodness of fit test for Hardy–Weinberg proportions is equivalent to the test for inbreeding, F = 0.
The inbreeding coefficient is unstable as the expected value approaches zero, and thus not useful for rare and very common alleles. For: E = 0, O > 0, F = −∞ and E = 0, O = 0, F is undefined.
Mendelian genetics were rediscovered in 1900. However, it remained somewhat controversial for several years as it was not then known how it could cause continuous characteristics. Udny Yule (1902) argued against Mendelism because he thought that dominant alleles would increase in the population. The American William E. Castle (1903) showed that without selection, the genotype frequencies would remain stable. Karl Pearson (1903) found one equilibrium position with values of p = q = 0.5. Reginald Punnett, unable to counter Yule's point, introduced the problem to G. H. Hardy, a British mathematician, with whom he played cricket. Hardy was a pure mathematician and held applied mathematics in some contempt; his view of biologists' use of mathematics comes across in his 1908 paper where he describes this as "very simple".
- To the Editor of Science: I am reluctant to intrude in a discussion concerning matters of which I have no expert knowledge, and I should have expected the very simple point which I wish to make to have been familiar to biologists. However, some remarks of Mr. Udny Yule, to which Mr. R. C. Punnett has called my attention, suggest that it may still be worth making...
- Suppose that Aa is a pair of Mendelian characters, A being dominant, and that in any given generation the number of pure dominants (AA), heterozygotes (Aa), and pure recessives (aa) are as p:2q:r. Finally, suppose that the numbers are fairly large, so that mating may be regarded as random, that the sexes are evenly distributed among the three varieties, and that all are equally fertile. A little mathematics of the multiplication-table type is enough to show that in the next generation the numbers will be as (p+q)2:2(p+q)(q+r):(q+r)2, or as p1:2q1:r1, say.
- The interesting question is — in what circumstances will this distribution be the same as that in the generation before? It is easy to see that the condition for this is q2 = pr. And since q12 = p1r1, whatever the values of p, q, and r may be, the distribution will in any case continue unchanged after the second generation
The principle was thus known as Hardy's law in the English-speaking world until 1943, when Curt Stern pointed out that it had first been formulated independently in 1908 by the German physician Wilhelm Weinberg. William Castle in 1903 also derived the ratios for the special case of equal allele frequencies, and it is sometimes (but rarely) called the Hardy–Weinberg–Castle Law.
Derivation of Hardy's equations 
The derivation of Hardy's equations is illustrative. He begins with a population of genotypes consisting of pure dominants (AA), heterozygotes (Aa), and pure recessives (aa) in the relative proportions p:2q:r with the conditions noted above, that is,
Rewriting this as (p + q) + (q + r) = 1 and squaring both sides yields Hardy’s result:
Hardy’s equivalence condition is
for generations after the first. For a putative third generation,
Substituting for p1 and q1 and factoring out (p + q)2 yields,
The quantity in brackets is equal to 1, therefore, p2 = p1 and will remain so for succeeding generations. The result will be the same for the other two genotypes.
Numerical example 
An example computation of the genotype distribution given by Hardy's original equations is instructive. The phenotype distribution from Table 3 above will be used to compute Hardy's initial genotype distribution. Note that the p and q values used by Hardy are not the same as those used above.
As checks on the distribution, compute
For the next generation, Hardy's equations give,
Again as checks on the distribution, compute
which are the expected values. The reader may demonstrate that subsequent use of the second-generation values for a third generation will yield identical results.
Graphical representation 
It is possible to represent the distribution of genotype frequencies for a bi-allelic locus within a population graphically using a de Finetti diagram. This uses a triangular plot (also known as trilinear, triaxial or ternary plot) to represent the distribution of the three genotype frequencies in relation to each other. Although it differs from many other such plots in that the direction of one of the axes has been reversed.
The curved line in the above diagram is the Hardy–Weinberg parabola and represents the state where alleles are in Hardy–Weinberg equilibrium.
It is possible to represent the effects of Natural Selection and its effect on allele frequency on such graphs (e.g. Ineichen & Batschelet 1975)
References and notes 
- Castle, W. E. (1903). "The laws of Galton and Mendel and some laws governing race improvement by selection". Proc. Amer. Acad. Arts Sci. 35: 233–242.
- Crow, Jf (Jul 1999). "Hardy, Weinberg and language impediments". Genetics 152 (3): 821–5. ISSN 0016-6731. PMC 1460671. PMID 10388804.
- Edwards, A.W.F. 1977. Foundations of Mathematical Genetics. Cambridge University Press, Cambridge (2nd ed., 2000). ISBN 0-521-77544-2
- Emigh, T.H. (1980). "A comparison of tests for Hardy–Weinberg equilibrium". Biometrics 36 (4): 627–642. doi:10.2307/2556115. JSTOR 2556115.
- Ford, E.B. (1971). Ecological Genetics, London.
- Guo, Sw; Thompson, Ea (Jun 1992). "Performing the exact test of Hardy-Weinberg proportion for multiple alleles". Biometrics (Biometrics, Vol. 48, No. 2) 48 (2): 361–72. doi:10.2307/2532296. ISSN 0006-341X. JSTOR 2532296. PMID 1637966.
- Hardy, G. H. (Jul 1908). "Mendelian Proportions in a Mixed Population". Science 28 (706): 49–50. doi:10.1126/science.28.706.49. ISSN 0036-8075. PMID 17779291.
- Ineichen, Robert; Batschelet, Eduard (1975). "Genetic selection and de Finetti diagrams". Journal of Mathematical Biology 2: 33. doi:10.1007/BF00276014.
- Masel, Joanna (2012). "Rethinking Hardy–Weinberg and genetic drift in undergraduate biology". BioEssays. doi:10.1002/bies.201100178.
- Pearson, K. (1903). "Mathematical contributions to the theory of evolution. XI. On the influence of natural selection on the variability and correlation of organs". Philosophical Transactions of the Royal Society of London, Ser. A 200 (321–330): 1–66. doi:10.1098/rsta.1903.0001.
- Stern, C. (1943). "The Hardy–Weinberg law". Science 97 (2510): 137–138. doi:10.1126/science.97.2510.137. JSTOR 1670409. PMID 17788516.
- Weinberg, W. (1908). "Über den Nachweis der Vererbung beim Menschen". Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg 64: 368–382.
- Wigginton, Je; Cutler, Dj; Abecasis, Gr (May 2005). "A Note on Exact Tests of Hardy-Weinberg Equilibrium". American Journal of Human Genetics 76 (5): 887–93. doi:10.1086/429864. ISSN 0002-9297. PMC 1199378. PMID 15789306.
- Yule, G. U. (1902). "Mendel's laws and their probable relation to intra-racial heredity". New Phytol 1 (193–207): 222–238. doi:10.1111/j.1469-8137.1902.tb07336.x.
- Hartl DL, Clarke AG (2007) Principles of population genetics. Sunderland, MA: Sinauer
- Emigh, Ted H. (1980). "A Comparison of Tests for Hardy–Weinberg Equilibrium". Biometrics 36 (4): 627–642. doi:10.2307/2556115. JSTOR 2556115.
- Crow, James F. (1999). "Hardy, Weinberg and language impediments". Genetics 152 (3): 821–825. PMC 1460671. PMID 10388804.
- Stern, Curt (1962). "Wilhelm Weinberg". Genetics 47: 1–5.
- EvolutionSolution (at bottom of page)
- Hardy–Weinberg Equilibrium Calculator
- genetics Population Genetics Simulator
- HARDY C implementation of Guo & Thompson 1992
- Source code (C/C++/Fortran/R) for Wigginton et al. 2005
- Online de Finetti Diagram Generator and Hardy–Weinberg equilibrium tests
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