Founder variant

A founder variant is a pathogenic variant inherited from a common ancestor together with the surrounding stretch of chromosome, and found at unusually high frequency in a population descended from a small founding group (Ahmad et al., Hereditary Cancer in Clinical Practice 2023). The NHGRI describes the underlying founder effect as "the reduction in genomic variability that occurs when a small group of individuals becomes separated from a larger population". The evidence that a recurring variant is a founder allele rather than the same mutation arising repeatedly is a shared haplotype: the markers flanking the variant travel with it, as Neuhausen and colleagues showed for BRCA1 in 1996 and Laitman and colleagues showed again for BRCA1 c.68_69delAG in 2013, dating it to roughly 750 to 1,500 years ago.

Why it matters practically: where three variants account for about 90 percent of the BRCA alterations found in a population, a three-variant panel costing a few pounds can do what full-gene sequencing does, which is what makes population-wide testing affordable. Israel used exactly this to become the first country to offer BRCA founder testing to a whole population group, and the Israeli cohort of 8,195 healthy Ashkenazi men gave the risk estimates that justified it (Gabai-Kapara et al., PNAS 2014).

What a founder variant is not. It is not ethnicity as destiny: it is one allele with a history, carried by some individuals and not others, and the same variant turns up on non-founder haplotypes in unrelated families. A negative founder panel does not exclude inherited risk, because most pathogenic variants in the same genes are not founders; in 250 ethnically diverse high-risk Israeli families screened after the common founders were excluded, 19 different pathogenic BRCA1/2 variants were found and none recurred often enough to panel for. Carrier frequencies are estimates from particular cohorts and differ between them, and penetrance measured in families referred to clinics is higher than penetrance measured in the general population. The clinical consequence is that a founder panel is a cheap first pass, not a substitute for sequencing.