Anextremely polymorphic locus ontheshort armofthehumanX chromosome withhomology tothelong armoftheY chromosome

A genomic DNA clonenamedCRI-S232 reveals anarrayofhighly polymorphic restriction fragments ontheX chromosome aswellasasetofnon-polymorphic fragments ontheY chromosome. Everyindividual hasmultiple bands, highly variable inlength, ineveryrestriction enzymedigest tested. Onesetofbandsisfoundinall males, andco-segregates withtheY chromosome infamilies. Thesesequences havebeen regionally localized bydeletion mappingtothelongarmoftheY chromosome. Segregation analysis infamilies showsthatalloftheremaining fragments co-segregate asasingle locus ontheX chromosome, eachhaplotype consisting ofthreeormore polymorphic fragments. Thislocus (designated DXS278)islinked toseveral markerson Xp,theclosest beingdic56(DXS143) atadistance of2cM Although itisoutside the pseudoautosomal region, theS232X chromosome locus showslinkage topseudoautosomal markersinfemale meiosis. Indetermining theX chromosome S232haplotypes of138offspring among19 families, weobserved threenon-parental haplotypes. Twowererecombinant haplotypes, consistent withacross-over amongtheS232-hybridizing fragments inmaternal meiosis. Thethird wasamutanthaplotype arising onapaternal X chromosome. Thelocus identified byCRI-S232 maytherefore bearecombination andmutation hotspot.