The Hardy-Weinberg law uses the binomial equation of p2 1 2pq 1 q2 5 1 to correlate allele, genotype, and phenotype frequencies in a very large ideal population. In this ideal population, individuals mate at random, no new mutations appear, no individuals enter or leave, and there are no genotype-dependent differences in fitness. In the binomial equation, p represents the frequency of one allele and q represents the frequency of the other, and p2, 2pq, and q2 represent the frequencies of the two homozygous and one heterozygous genotypes.
A population satisfying the Hardy-Weinberg assumptions is said to be at Hardy-Weinberg equilibrium. In such a population, allele frequencies remain constant from one generation to the next, and the genotype frequencies of p2, 2pq, and q2 appear in one generation, after which they are maintained.
Evolution consists of changes in allele frequency over time. Selection acting on genotype-dependent differences in fitness can drive evolution. Selection does not entirely eliminate deleterious recessive alleles from a population. One reason for this is heterozygous advantage.
The existence of an evolutionary equilibrium is another reason deleterious recessive alleles persist in populations. The evolutionary equilibrium is a balance between mutation to a new allele and selection against that allele. Late onset of a disease undermines selection against the disease allele, while genetic drift has an unpredictable effect on the evolutionary equilibrium.
For quantitative traits, the environmental variance is a measure of the influence of environment on phenotypic variation. Similarly, genetic variance measures the contribution of genes to phenotypic variation. Total phenotype variance is the sum of genetic variance and environmental variance.
Measures of environmental, genetic, and total phenotype variance make it possible to define the heritability of a trait as the proportion of total phenotype variance attributable to genetic variance. With traits for which the number and identity of contributing genes remain unknown and there is no way to obtain genetic clones, it is possible to correlate phenotypic variation with the genetic relatedness of individuals--that is, the average fraction of common alleles at all gene loci that the individuals share because they inherited them from a common ancestor--to measure the heritability of a trait.
To ascertain the heritability of a human trait, population geneticists often turn to studies of twins. The most useful approach is to compare the phenotypic differences between pairs of monozygotic and dizygotic twins. Environmental changes can always influence heritability.
The heritability of a trait quantifies its potential for sustaining selection and thus its potential for evolution from one generation to the next.
Variations in polygenic traits arise at a rapid rate through selection because changes at many loci contribute to changes in phenotype. Nevertheless, it is possible that many polygenic traits are determined by tens, rather than thousands, of loci.
