Chromosome preparations from seven subjects with aberrations of sex chromosomes were utilized for in situ hybridization studies with the tritium-labeled Y-derived probe p50f. Two subjects had a pseudodicentric chromosome consisting of two copies of Yp and a portion of Y long arm; two were XX males [46,XX,t(Xp;Yp)], one was missing part of the Y short arm, and another had t(5p;Yq); in addition cells from an XYY male as well as a normal 46,XY male, and a 46,XX female, were hybridized with the same probe. The hybridization technique of Harper and Saunders (1981) was used. There was excess labeling of the Yp/paracentromeric regions in the cases with the normal Y, the XYY, the pseudodicentric Y, and the 5/Y translocation. No significant label was seen on metaphases from the normal 46,XX female or the female with the partially missing Y short arm. Excess label was present on the X short arm in the cases of the XX males; there were 8% and 9.5% of cells with label. The combined cytogenetic and hybridization data indicate that one X short arm in these XX males has undergone a translocation with Yp, and that genes for sex determination probably reside on the distal half of the Y short arm.
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