Polydactyly showing 65–90% expression is an example of
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 a
Ref: Hartl, Essential Genetics, Chapter 5: Incomplete Penetrance Example of Polydactyly
