The unclear pattern of inheritance of postaxial polydactyly prompted this search for evidence of imprinting or change of expression in males and females using material of the Latin American Collaborative Study of Congenital Malformations. The frequency of affected offspring for 196 fathers with polydactyly was compared with that for 233 mothers with the same condition, stratified according to African and non-African ancestry. The postaxial polydactyly prevalence rate among the offspring of affected black fathers (44%) was larger than that in the group of affected black mothers (31%), with no difference between affected nonblack fathers (34%) and affected nonblack mothers (33%). The sex ratio (.51) observed in 631 black propositi and in 829 nonblack propositi with polydactyly (.58) could be a further indication of etiologic heterogeneity for polydactyly between these two ethnic groups. The segregation distortion in favor of affected among the offspring of affected black fathers could be interpreted as the effect of a sex-linked recessive modifier gene acting during gametogenesis on an autosomal dominant polydactyly gene, this modifier being more frequent in Africans.