Is a case of genetic drift in which rare alleles, or combinations of alleles, occur in higher frequency in a population isolated from the general population.

Founder effect

Explanation:

In founder effect , a small population get separated from the larger ancestral population which undergo strong genetic drift in order to diverge the founder population from the ancestral population through combination of allele or rare alleles at a very high frequency.

There is a lack of genetic diversity and hence the genetic variation as the mating population size is small.

For example - the incidence of occurrence of fumarase deficiency in a population of members of a  church.

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