The term "founder effect" refers to the observation that when a small group of individuals breaks off from a larger population and establishes a new population, chance plays a large role in determining which alleles are represented in the new population. The particular alleles may not be representative of the larger population. As the new population grows, the allele frequencies will usually continue to reflect the original small group.

Genetic Characteristics of Founder Populations

Because the founder population is small, genetic drift can play an important role in determining the genetic makeup of subsequent generations, and allele frequencies may fluctuate. For example, consider an extreme situation where a new population is founded by just two individuals, a male and a female, perhaps because they are stranded on an island. Assume that the mother is heterozygous for a particular allele (Aa), while the father is homozygous (AA...