Genetic structures of Populations

The genetic structure refers to the frequencies of the alleles of a certain population.

If the phenotype is observed, only the genotype of the homozygous recessive alleles can be known; the calculations provide an estimate of the remaining genotypes. Since each individual carries two alleles per gene, if the allele frequencies (p and q) are known, predicting the frequencies of these genotypes is a simple mathematical calculation to determine the probability of getting these genotypeswhen two alleles are drawn at random from the gene pool. So 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 producing green peas (Figure below). In other words, the frequency of pp individuals is simply p2; the frequency of pq individuals is 2pq; and the frequency of qq individuals is q2. And, again, if p and q are the only two possible alleles for a given trait in the population, these genotype frequencies will sum to one: p2 + 2pq + q2 = 1.

First section demonstrates how to obtain p and q values that represent allelic frequencies. One yellow circle with two capital Y letters, number of individuals out of 500 is 245, contributes 490 alleles to gene pool with total of 1000 alleles and genotypic frequency is 0.49. One yellow circle heterozygous for capital Y and lower case y, number of individuals out of 500 is 210, contributes 210 capital Y alleles and 210 lowercase y alleles. One green circle homozygous for lowercase y, number of individuals is 45 out of 500, contributes 90 lowercase y alleles. Allelic frequency of capital Y is 0.7, represented by p. Allelic frequency of lowercase y is 0.3, represented by q. Second section of chart is a punnett's squares table with 4 quadrants. Top left quadrant contains yellow circle around two capital Y alleles. p square equals .49. Top right quadrant contains yellow circle around one capital Y and one lowercase y allele. pq equals 0.21. Bottom left quadrant contains yellow circle around one capital Y allele and one lowercase y allele. pq equals 0.21. Lower right quadrant contains green circle around two lowercase y alleles. q squared equals 0.09. Below the Punnett's square p squared plus 2pq plus q squared equals 1. Below this 0.7 squared plus 2 times 0.7 times 0.3 plus 0.3 squared equals 1. Below this 0.49 plus 0.42 plus 0.09. 0.49 is the predicted frequency of homozygous capital Y genotypes. 0.42 is the predicted frequency of heterozygous genotypes. 0.09 is the predicted frequency of homozygous y genotypes.

When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from generation to generation, and the distribution of alleles can be determined from the Hardy-Weinberg equation. If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play.

Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction.

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution.