A prospective psychological and cytogenetic study of three girls with mosaic mongolism.

Three girls with mosaic mongolism (46, XX/47, XX, +21) were followed cytogenetically and psychologically from the time of diagnosis at 8, 17 and 32 months of age, respectively, to the age of 13--15.5 years. All showed muscular hypotonia and hyperflexible joints at the time of diagnosis, but otherwise the physical characteristics of Down's syndrome were weakly expressed. The percentage of trisomic cells in the peripheral blood decresed with time, but were still higher in lymphocytes than in skin fibroblasts at the last investigation. Developmental milestones were delayed in all cases, and the developmental and intelligence quotients were decreased. Mental retardation was only slight in one of the cases (I.G. = 65 at the age of 14 years 8 months). The intelligence quotients showed declining trends with time. Social ability and school results tended to be better than could be expected from the test results.