Familial 'partial 9p' trisomy: six cases and four carriers in three generations.

Six cases of translocation trisomy for the distal half of the short arm of a number 9 chromosome and four asymptomatic balanced translocation carriers are presented in a three-generation pedigree. The clinical features are remarkably similar to those recently recognized and increasingly reported in full short arm (9p) trisomy and should be considered a modification of the same syndrome. In addition to non-specific mental retardation and short stature, there is, in common, a characteristic facies, including down-turned corners of the mouth, a slightly bulbous nose, moderately large ears, suggestively wide-set eyes with an antimongoloid slant, dysplasia and hypolasis of the nails, clindactyly of the 5th fingers, and abnormal dermatoglyphs. It appears that the 'trisomy 9p syndrome' in its variant forms, including trisomies for more or less than just the short (p) arm, is one of the most common clinical autosome anomalies in humans, exceeded only by trisomy 21 (Down's syndrome) and possibly trisomies of chromosomes 13 and 18.