Familial caudal dysgenesis: evidence for a major dominant gene.

Four sibs with varying degrees of caudal dysgenesis are described. Case 1 showed aberrant umbilical cord vasculature with a single umbilical artery near the placental insertion. Cases 2 and 3 showed full sirenomelia, one with a complex congenital heart defect. Case 4 had an imperforate anus and an excessively long umbilical cord. The father's half-sib had an imperforate anus, rectovaginal fistula and genitourinary anomalies. A dominant gene with reduced penetrance is likely.