No abstract

SUMMARY Cytogenetic studies on a mentally retarded boy revealed an X-Y translocation, karyotype 46,X,t(X;Y)(p22;ql 1). Only 5 other such cases have been reported and these were all females. The unequivocal male phenotype suggested non-random inactivation of the normal maternally derived X chromosome, and that the non-inactivated X-Y translocation chromosome included the locus for male determination. Confirmation of this was provided by unassociated X and Y chromatin in interphase cells, as well as by reverse banding after BrdU incorporation and autoradiography of metaphase chromosomes. There was anomalous Xg blood group inheritance in the proband, indicating possible localisation of the Xg locus to the terminal portion of the X short arm. Linkage of Xg and a form of X-linked mental retardation is suggested. Close linkage of the Xg locus with the loci for aPgalactosidase, phosphoglycerate kinase, G-6-PD, and MPS II was excluded.Cytologically identifiable translocation of the Y chromosome to the X chromosome has been reported very rarely (Khudr et al., 1973;Borgaonkar et al., 1974; Van den Berghe et al., 1977). In all instances where there was no free-lying Y chromosome, the patients were phenotypic females. Two XX males with presumptive morphological evidence of a Y to X translocation have also been documented (Madan and Walker, 1974;Wachtel et al., 1976).We wish to report the investigation of a mentally retarded child with a morphologically identified X-Y translocation and an unequivocally male phenotype. This patient afforded a valuable opportunity for studying the pattern of X inactivation in such cases, as well as the location of the male determining genes of the Y chromosome, and the linkage of genes possibly located on the distal portion of the short arm of the X chromosome, including the Xg locus. Case reportThe proband was first referred at the age of 4 years for investigation of severe mental retardation associated