A cytogenetic survey of an institution for the mentally retarded. II. C-band chromosome heteromorphisms.

Heteromorphisms of chromosomes 1, 9, and 16 were studied by C-banding in a population of 403 mentally retarded individuals from diverse ethnic groups. A significant difference in the distribution of heteromorphisms was found among the different racial groups. The Orientals had a larger C-band on chromosome 1 and a smaller C-band on chromosome 9 than the other racial groups while the Caucasians had a larger C-band on chromosome 9. The Oriental group also had a significantly greater proportion of inverted C-bands. No differences were found in the distribution of C-band heteromorphisms among different etiologic categories of mental retardation.