FUNCTIONAL DISOMY OF XP22-PTER IN 3 MALES CARRYING A PORTION OF XP TRANSLOCATED TO YQ

A number of Xp22;Yq11 translocations involving the transposition of Yq material to the distal short arm of the X chromosome have been descri bed. The reciprocal product, i.e. the derivative Y chromosome resultin g from the translocation of a portion of Xp to Yq, has never been reco vered. We searched for this reciprocal product by performing dosage an alysis of Xp22-pter loci in 9 individuals carrying a non-fluorescent Y chromosome. In three mentally retarded and dysmorphic patients, dosag e analysis indicated the duplication of Xp22 loci. Use of the highly p olymorphic probe CRI-S232 demonstrated the inheritance of paternal Xp- specific alleles in the probands. In situ hybridization, performed in one case, confirmed that 29CL pseudoautosomal sequences were present, in addition to Xpter and Ypter, in the telomeric portion of Yq. To our knowledge, these are the first cases in which the translocation of Xp material to Yq has been demonstrated. The X and Y breakpoints were ma pped in the three patients by dosage and deletion analysis. The X brea kpoint falls, in the three cases, in a region of Xp22 that is not reco gnized as sharing sequence similarities with the Y chromosome, thus su ggesting that these translocations are not the result of a homologous recombination event.