Founder effects occur when a population is established by a small number of individuals from a larger population and remain somewhat isolated. This small group of colonists may be carrying genes that are uncommon in the original larger population, which will now become common among the new colonists’ descendants. Other genes present in the original population, however, may be absent from the new group altogether.
For example, Huntington’s Disease is prevalent in the Afrikaner; or Dutch-descent population; of South Africa because a gene for Huntington’s Disease happened to be unusually common among the small group of original Dutch colonists.
Another example of the founder effect is the unusually high frequency of Huntington’s Disease in a region in Northern Venezuela called Lake Maracaibo. Lake Maracaibo is a large lake where people live in small fishing villages situated around the lake. These people are fairly isolated from the populations beyond the shores of the lake. Because the incidence of HD is so high in this area, 700 cases per 100,000, there are pedigrees for many of the families.
If these pedigrees are traced back, it appears that a woman came to Lake Maracaibo about 200 years ago and brought the Huntington’s gene into this population. This woman had 10 children and because the population is so isolated, the gene has persisted in an unusually high frequency.