Genetics of acheiropodia ("The handless and footless families of Brazil"): IV. Sex ratio, consanguinity and birth order.

The sex ratio among acheiropods is shown to be not significantly different from the sex ratio among their normal sibs, as well as among other segments of the family (parental sibships, cousin sibships and other sibships). The frequency of consanguineous marriages among the parents of acheiropods (82%) is one of the highest thus far reported among parents of individuals affected with rare autosomal recessive anomalies or diseases. No evidence was found of any overall positive or negative birth order effect on the incidence of acheiropodia. These results further strengthen the hypothesis of a simple genetic mechanism for acheiropodia, namely autosomal recessive inheritance, without any detectable complication as regards sex ratio, consanguinity and birth order. Although the family size from heterozygous parents (producing at least one acheiropod) was twice as large as family sizes in other segments of the families (8 against 4), we have at present no evidence to support either heterozygous advantage or reproductive compensation.