Population Genetics

All of the alleles shared by all of the individuals in a population make up the population's gene pool. In diploid organisms such as humans, every gene is represented by two alleles. The pair of alleles may differ from one another, in which case it is said that the individual is "heterozygous" for that gene. If the two alleles are identical, it is said that the individual is "homozygous" for that gene. If every member of a population is homozygous for the same allele, the allele is said to be fixed. Most human genes are fixed and help define humans as a species.

The most interesting genes to geneticists are those represented by more than one allele. Population genetics looks at how common an allele is in the whole population and how it is distributed. Imagine, for example, an allele "b" that when homozygous, "bb," produces blue-eyed individuals. Allele b A caribou herd is a population: a group of organisms of the same species that inhabit an area and interbreed. Individuals may migrate between populations, causing gene flow. might have an overall frequency in the population of 20 percent; that is, 20 percent of all the eye-color alleles are b.

However, not everyone who has the b allele will be homozygous for b. Some people will have b combined with another allele, "B," which gives them brown eyes (because B is dominant and b is recessive). Others won't have the b allele at all and instead will be homozygous for B.

The frequency of each genotype—whether bb, Bb, or BB—in the population is also of interest to population geneticists. The frequency of alleles and genotypes is called a population's genetic structure. Populations vary in their genetic structure. For example, the same allele may have a frequency of 3 percent among Europeans, 10 percent among Asians, and 94 percent among Africans. Blood types vary across different ethnic groups in this way. The frequency of genotypes depends partly on the overall allele frequencies, but also on other factors.