recap

20.3 recap

Hardy–Weinberg equilibrium describes the theoretical conditions required for evolution not to occur. Deviations from Hardy–Weinberg expectations provide information about how evolution is occurring in a given population.

learning outcomes

You should be able to:

  • Calculate the allele frequencies for two alleles at a given locus in a diploid population.

  • Calculate observed and expected genotype frequencies from allele frequencies based on the principles of Hardy–Weinberg equilibrium.

  • Analyze data and formulate hypotheses to explain deviations from Hardy–Weinberg expectations in observed genotype frequencies.

Question 1

The following sample lists the genotype at locus A for 10 individuals in a diploid population: AA, AA, Aa, Aa, Aa, Aa, aa, aa, aa, aa.

  1. Based on this sample, what is the observed frequency of allele a? The observed frequency of allele A?

  2. What are the observed frequencies of genotypes aa, Aa, and AA?

  3. After one generation of random mating, what would be the Hardy–Weinberg expectations for the frequencies of genotypes aa, Aa, and AA?

  1. Frequency of allele a: 0.60; of allele A: 0.40;

  2. Frequency of genotype aa: 0.40; of genotype Aa: 0.40; of genotype AA: 0.20;

  3. Expected frequency of genotype aa: 0.36; of genotype Aa: 0.48; of genotype AA: 0.16

Question 2

In a population of toads, allele A is present at a frequency of 0.2, and allele a is present at a frequency of 0.8. Based on Hardy–Weinberg expectations, what is the expected frequency of the genotype Aa in the population?

2pq = 2(0.2)(0.8) = 0.32

Since the observed frequency of heterozygotes is lower than the frequency predicted by Hardy–Weinberg expectations, any of the following are reasonable explanations of this pattern:

  • Toads from two or more adjacent subpopulations could be breeding in the same pond. The frequencies of alleles in each population may differ, resulting in high frequencies of different alleles in each subpopulation. In other words, the assumption of no gene flow has been violated.

  • Toads may not be breeding randomly within the population. For example, if closely related toads are more likely to mate with each other than with distantly related toads, then fewer heterozygotes would be expected in population (a form of inbreeding).

  • Heterozygous individuals may be at a disadvantage in the population (e.g., fewer heterozygotes may survive to adulthood). This would represent a violation of the Hardy–Weinberg assumption of no selection.

Question 3

Assume that the observed frequency of the genotype Aa in the toad population in Question 2 is 0.15, rather than the expected frequency you calculated. What are some possible explanations?

Presumably, the observed population is large enough to calculate that the low frequency of heterozygotes is significant. However, if the present generation of toads was produced by a small number of parents in the previous generation, then random effects of a small population size could explain the observed pattern.

Our discussion so far has focused on changes in allele frequencies at a single gene locus. Genes do not exist in isolation, however, but interact with one another (and with the environment) to produce an organism’s phenotype. What effects can these interactions have on selection?