founder mutation
A founder mutation is a specific disease-causing change in DNA that is unusually common in a particular population because that population grew from a small group of ancestors, one or a few of whom happened to carry it. Picture a small band of settlers founding a new community: if one settler carried a rare mutation, then by chance that single copy can become the seed for many descendants generations later.
Founder mutations arise from the founder effect, a form of genetic drift in which the gene pool of a new population reflects only the few founders, not the larger group they came from. A mutation that is rare worldwide can therefore be relatively frequent in a founder population, and very often it is exactly the same shared mutation rather than many different ones, because the descendants inherited the founder's identical DNA neighborhood (haplotype) around it.
Recognizing founder mutations is practically useful: in a population with a known founder allele, a genetic test can focus first on that one variant, making screening cheaper and faster. Examples include particular BRCA1 and BRCA2 cancer-risk mutations in Ashkenazi Jewish populations and the Tay-Sachs alleles concentrated in the same group.
Because three specific BRCA mutations account for most hereditary breast and ovarian cancer risk in one founder population, testing can begin by checking just those three variants.
A known founder allele lets testing target one variant first, saving time and cost.
A telltale sign of a founder mutation is that affected people share not just the same mutation but the same surrounding haplotype, evidence that they all descend from a single ancestral chromosome.