Practice question

Polydactyly extra digits provides classic human example of incomplete penetrance with dominant allele showing 65-90% expression. In family surveys, 10-35% of individuals who inherit pathogenic variant remain phenotypically normal with five digits, despite carrying mutation. Developmental buffering, modifier loci, and limb patterning threshold variability explain non-penetrance. Consequently unaffected parents can transmit allele to affected children, mimicking recessive inheritance or new mutation. Penetrance value derived from population pedigrees aids counselors calculating recurrence risk and explaining why generation skipping does not rule out dominant transmission.