Cytological mapping of the human glucose-6-phosphate dehydrogenase gene distal to the fragile-X site suggests a high rate of meiotic recombination across this site.

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Abstract
The human gene for glucose-6-phosphate dehydrogenase (G6PD) was subregionally mapped to band Xq28 by segregation analysis in rodent-human somatic cell hybrids. A common type of X-linked mental retardation associated with an inducible fragile site at Xq27-Xq28 segregates in a close linkage relationship with a G6PD variant, but the relative position of G6PD with respect to the fragile site was not yet established. This fragile-X syndrome was shown to be closely linked also to a Taq I restriction fragment length polymorphism detected by a c[complementary]DNA probe for factor IX, and the latter locus was mapped to the subtelomeric region Xq26-Xq28. The in situ hybridization studies reported here provide strong evidence that G6PD is located on the Xq telomeric fragment distal to the fragile site. These observations and the well-established knowledge that the genes for Deutan and Protan color blindness are closely linked to G6PD, but segregate independently of factor IX deficiency, suggest that the fragile site associated with this type of X-linked mental retardation occurs in a region prone to high frequency of meiotic recombination.