As a result, the only way to determine the exact distribution of phenotypes in a population is to go out and count them. Populations in nature are constantly changing in genetic makeup due to drift, mutation, possibly migration, and selection. Of course, even Hardy and Weinberg recognized that no natural population is immune to evolution. In theory, if a population is at equilibrium-that is, there are no evolutionary forces acting upon it-generation after generation would have the same gene pool and genetic structure, and these equations would all hold true all of the time. Plants with VV or Vv genotypes would have violet flowers, and plants with the vv genotype would have white flowers, so a total of 480 plants would be expected to have violet flowers, and 20 plants would have white flowers. The expected distribution is 320 VV, 160Vv, and 20 vv plants. Again, if p and q are the only two possible alleles for a given trait in the population, these genotypes frequencies will sum to one: p 2+ 2pq + q 2 = 1. In other words, the frequency of pp individuals is simply p 2 the frequency of pq individuals is 2pq and the frequency of qq individuals is q 2. In the above scenario, an individual pea plant could be pp (YY), and thus produce yellow peas pq (Yy), also yellow or qq (yy), and thus produce green peas (Figure 1). Since each individual carries two alleles per gene, if we know the allele frequencies (p and q), predicting the genotypes’ frequencies is a simple mathematical calculation to determine the probability of obtaining these genotypes if we draw two alleles at random from the gene pool. The calculations provide an estimate of the remaining genotypes. If we observe the phenotype, we can know only the homozygous recessive allele’s genotype. However, what ultimately interests most biologists is not the frequencies of different alleles, but the frequencies of the resulting genotypes, known as the population’s genetic structure, from which scientists can surmise phenotype distribution. In other words, all the p alleles and all the q alleles comprise all of the alleles for that locus in the population. If these are the only two possible alleles for a given locus in the population, p + q = 1. The variable p, for example, often represents the frequency of a particular allele, say Y for the trait of yellow in Mendel’s peas, while the variable q represents the frequency of y alleles that confer the color green. Working under this theory, population geneticists represent different alleles as different variables in their mathematical models. While no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes. The Hardy-Weinberg principle assumes conditions with no mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population. The theory, which later became known as the Hardy-Weinberg principle of equilibrium, states that a population’s allele and genotype frequencies are inherently stable- unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg stated the principle of equilibrium to describe the population’s genetic makeup.
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