gene mutation observed in high frequency in a specific population due to
the presence of that gene mutation in a single ancestor or small number of
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
The founder effect is the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony. The new population may be very different from the original population, both in terms of its genotypes and phenotypes. In some cases, the founder effect plays a role in the emergence of new species.
Definition from: Talking Glossary of Genetic Terms from the National Human Genome Research Institute
See also Understanding Medical Terminology.