Assignment of the locus order DXS28-DXS67-DMD as a spin-off from diagnostic DNA marker analysis in a family with Duchenne muscular dystrophy.

During diagnostic segregation analysis for seven DNA markers, linked to and flanking the locus for Duchenne muscular dystrophy (DMD), a family was identified in which a boy with a recombinant X chromosome had inherited his maternal grandmother's alleles at the loci DXS43 (D2/Pvu II) and DXS28 (C7/Eco RV), and his maternal grandfather's alleles at DXS67 (B24/Msp I) and DXS84 (754/Pst I). Combined with earlier data this finding strongly suggests the locus order DXS28-DXS67-DMD. Another recombination event, identified in the same family, supports the previously established order DMD-DXS84-OTC. The diagnostic importance of flanking markers, and the likelihood of false diagnostic conclusions due to possible double crossovers, with and without demonstrable neighbouring single crossover events, are discussed.