Uniparental Origin of Sex Chromosome Polysomies

The parental origin of the additional sex chromosomes in 8 cases with high-order sex chromosome polysomies was determined using DNA polymorphisms. The additional sex chromosomes were paternally derived in 3 48, XXYY cases, and maternal in origin in 1 48, XXXY case and 4 49, XXXXY cases. Thus, all extra chromosomes, within a particular patient, were always derived from only one parent. Their most likely origin was successive nondisjunction at the first and second meiotic division in one germ cell. The mechanism involved remains unclear, but appears to be independent of parental ages.