Founder effect
The positive effect on gene frequency when a population (a colony) has only a small number of original settlers (founders) one or more of whom had that gene.
For example, the gene for Huntington disease was introduced into the Lake Maracaibo region in Venezuela early in the 19th century.
So there are now over a hundred persons with Huntington disease and at least 900 persons at risk for that deadly disease in that region, the largest known aggregation with the Huntington gene in the world.