Founder effect

A founder effect occurs when a small subset of individuals establishes a new population, and that subset carries (by chance) allele frequencies that differ from the parent population. Rare alleles can become common, or be lost entirely, in a founding population. The founder effect explains why certain disorders are enriched in specific ancestries and is the basis for targeted carrier screening panels.

• A founding group is a small, non-random sample of the parent population's genetic diversity.
• Rare alleles carried by founders become disproportionately frequent in descendants.
• Allele frequencies drift further from the parent population over generations, especially if the new population remains isolated and endogamous.
• Distinguished from genetic drift broadly by occurring at a specific founding event rather than continuously.

• Ashkenazi Jewish: enriched for Tay-Sachs disease (HEXA), Gaucher disease type 1 (GBA), Canavan disease (ASPA), familial dysautonomia (IKBKAP), BRCA1 (185delAG, 5382insC) and BRCA2 (6174delT) founder variants. Underpins the standard Ashkenazi carrier panel.
• French-Canadian (Quebec): tyrosinemia type I (Saguenay-Lac-Saint-Jean), autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS), familial hypercholesterolemia (French-Canadian LDLR deletion).
• Finnish: Finnish disease heritage of about 40 rare AR disorders, including aspartylglucosaminuria, congenital nephrotic syndrome (NPHS1), and cornea plana.
• Amish (Old Order): Ellis-van Creveld syndrome (EVC), glutaric aciduria type I (GCDH), maple syrup urine disease (BCKDHA).
• Afrikaners (South Africa): variegate porphyria (PPOX R59W), familial hypercholesterolemia.
• Native American (several groups): severe combined immunodeficiency (Athabaskan), cystic fibrosis (R1162X in Pueblo populations, including Zuni).

• Targeted carrier screening uses ancestry-specific founder variant panels to achieve high sensitivity with few assays. Expanded (pan-ethnic) NGS panels are supplanting ethnicity-specific panels but the founder-variant knowledge still guides variant interpretation and family testing.
• Recognizing founder-effect diseases aids diagnosis (ancestry-specific differential for unexplained findings) and carrier testing strategy (targeted first-pass, then sequencing if negative).
• Variant interpretation: founder variants have strong population data supporting pathogenicity; haplotype testing (presence of the founder haplotype) can confirm.
• Insurance and ethics: ancestry-directed screening raises issues of ethnicity self-identification, genetic privacy, and the increasing admixture of populations.