Among the conditions necessary for Hardy-Weinberg equilibrium were large populations, random mating, lack of selective advantage for any one genotype, and a lack of other factors that might affect allele frequency. In reality, it is infrequent that these conditions are met. As an example, consider random mating. Certainly among humans, random mating does not occur. We tend to mate (excuse the crude phraseology) with people with similar socioeconomic backgrounds, with similar education levels, and with similar interests.
Let's look at some of the factors that can affect allele frequency and thereby prevent Hardy-Weinberg equilibrium.
Factors affecting allele frequency
- Mutation. This creates new alleles of a gene. Mutation isn't a major factor, because it happens sufficiently rarely that the frequency of the new allele would be negligible unless other forces (such as natural selection) come into play. For example, if one person in a population of 1 million experiences a mutation in one copy of a gene, the frequency of that new allele would be 1 in 2 million, or 0.0000005, which is hardly worth considering.
- Migration. If individuals leave a population, or new individuals enter a population, this can change the allele frequency. Using the Drosophila example we examined earlier, if just 10 new individuals, all genotype AA, enter the population, then the frequency of allele A will change from 0.50 to 0.53.
- Natural selection. This is the major force that shifts allele frequency. Sometimes, having one particular allele will confer on an individual an increased chance of survival and reproduction. In such a case, that allele will increase in frequency. Alternatively, a particular allele can be detrimental to an individual, causing them to have a reduced chance of survival to reproduction. In this case, the allele will be reduced in frequency. An interesting example of this involves the sickle cell allele of the globin protein. Among Caucasians, who have historically lived primarily in the northern hemisphere, the frequency of the sickle cell allele is very low, because it causes anemia and is therefore a detriment to the individual. Among those of African descent, however, the frequency of this allele is much higher. This is because having one copy of the sickle cell allele confers resistance to malaria, the benefit of which outweighs the detriment of anemia.
- Genetic drift. This is a situation that may occur in small populations. Genetic drift is simply random fluctuations in allele frequency due to chance deviation. As an extreme example, let's consider two people stranded forever on a desert island, who decide to create a new population on the island. Both are heterozygous for a particular gene, and so should (according to probability) produce all possible genotypes. If however, due entirely to random chance, all of their offspring are homozygous recessive, then that population will eventually have only one allele of that gene - the recessive one. If this had happened in a large population, the few extra homozygous recessives wouldn't have made a dent in the allele frequency . Because the population is so small, however, in this case the effect is a major one. This is known as the founder effect, where a population is founded by a small number of individuals, so any random deviations from probability have a major impact on allele frequencies. In humans, this tends to happen with populations who have isolated themselves for religious/cultural reasons (such as the Amish).
- Inbreeding. This is mating with close genetic relatives, which is an extreme example of nonrandom mating. Studies have shown that inbreeding tends to increase homozygosity (both dominant and recessive). In this way, recessive traits that cause health problems tend to show up more frequently than in the random population. Such problems are fairly common in purebred dogs and thoroughbred horses. Inbreeding has a substantial effect on overall allele frequency only if the population is small.
Population Genetics: Summary of Key Points
- Genes and their effects can be studied on populations as well as individuals.
- Allele frequencies for a gene can be determined within a population. The frequencies of all alleles of a gene always adds up to 1. Frequencies of each genotype can also be calculated.
- The Hardy-Weinberg principle states that under a set of presumed conditions, genotype frequencies will be constant from one generation to another.
- The Hardy-Weinberg equation can be used to determine whether or not a population is in equilibrium with respect to a particular gene. If there is no equilibrium, then one of the conditions is not being met, and factors such as mutation, migration, natural selection, genetic drift, and inbreeding may be playing a role in the population.
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