Effect, founder
A population group with an unusual frequency of a gene due to there having been only a small number of original members ('founders') one or more of whom had that gene.
For example, the gene for Huntington disease was introduced into the Lake Maracaibo region of Venezuela early in the 19<SUP>th</SUP> century, so there are now over a hundred persons with Huntington diseaseand at least 900 persons at risk for that deadly disease in that region, the largest known aggregation known in the world with the Huntington gene