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A case of partial trisomy 9 is described in a mentally retarded and dysmorphic child, confirming that this chromosome unbalance results in a characteristic clinical entity. This trisomy arose through aberrant segregation of translocation chromosome during meiosis in the patient's mother, who is a balanced heterozygote for a complex translocation involving chromosomes 9, 21 and 22. The phenotypically normal sister of the proposition is also carrier of the same complex translocation.

Centric fission of chromosome No. 4 was found in the healthy mother of two children with trisomy 4p. The two telocentrics derived are stable and show no evidence of fusing again. The 2:3 ratio of unbalanced offsprings born to the proposita indicates that, contrary to the evidence emerging from studies in other species, the risk for the production of unbalanced gametes is high.