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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 a with frequencies f(A) = p and f(a) = q, respectively, the expected genotype frequencies are f(AA) = p2 for the AA homozygotes, f(aa) = q2 for the aa homozygotes, and f(Aa) = 2pq for the heterozygotes. The genotype proportions p2, 2pq, and q2 are called the Hardy-Weinberg proportions. [Note that the sum of all genotype frequencies of this case is the binomial expansion of the square of the sum of p and q, and such a sum, as it represents the total of all possibilities, must be equal to 1. Therefore (p q)2 = p2 2pq q2 = 1. The solution of this equation is q = 1 - p.] If union of gametes to produce the next generation is random, it can be shown that the new frequency f′ satisfies \textstyle f'(\text{A}) = f(\text{A}) and \textstyle f'(\text{a}) = f(\text{a}). That is, allele frequencies are constant between generations. This principle was named after G. H. Hardy and Wilhelm Weinberg, who first demonstrated it mathematically. 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. #model #GeneStructure #MolecularBiology #nonrandomMating #gene #genotype #locus #GeneticsLecture #GeneticExamQuestionsSolutions #theorem #RecurrentMutation #Selection #HardyWeinbergEquilibrium #alleleFrequencies #GeneticsExamQuestionsSolutions #HardyWeinbergPrinciple #alleleAndGenotypeFrequenciesInAPopulation #Genetics101 #genomics #mutation #Diploid #Inbreeding #law
