Brief report: Linkage between G6PD and fragile‐X syndrome

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Abstract
Eighteen Sardinian pedigrees segregating for the X‐fragile site syndrome were studied with respect to the segregation of the fragile site (FS) at Xq28, mental retardation, and macro‐orchidism. No exception was found in the association of this symptomatic triad (MOM‐X) in 41 out of 42 patients examined. The exceptional individual had micro‐ rather than macro‐orchidism and was found to have a 47, XXY sex chromosome complement. In six informative sibships, the MOM‐X syndrome was found to segregate in close linkage association with G6PD‐deficiency or protan colorblindness. The maximum likelihood estimate of recombination is 6% with 90% fiducial limits between 2.5 and 19.5% and an odds ratio in favor of measurable linkage of 428:1. However, no hint of measurable linkage was found in six pedigrees segregating for G6PD and the Renpenning syndrome or other unspecified types of X‐linked mental retardation. These data give strong support to the generally held hypothesis that the FS at Xq28, characteristic of the MOM‐X syndrome, is a direct expression of a genetic change in the same chromosomal region. They also clearly suggest that X‐linked MR without FS may be the result of different allelic mutations at the same locus.