| FRAGILE X MENTAL RETARDATION SYNDROME |
| LINKAGE MAPPING: Brown et al. (1988) performed a multilocus linkage analysis of the fragile X syndrome in 147 families using 4 flanking markers. As previously observed by Giannelli et al. (1987), significant variation in the recombination distance between F9 and FRAXA (fragile X, locus A) was found also in this group of families. Heterogeneity testing showed that 20% of the families had tight F9-FRAXA linkage, whereas 80% demonstrated loose linkage, with an average recombination distance of 0.35. On average, the multipoint distances found were DXS51-F9, 6.9%; F9-FRAXA, 22.4%; FRAXA-DXS52, 12.7%; and DXS52-DXS15, 2.2%. Thibodeau et al. (1988) also reported on linkage data between the fragile X locus and 4 polymorphic markers: DXS51, F9, DXS98, and DXS52. The markers were studied in 14 families with fragile X and 9 normal pedigrees from the CEPH collection. In this set of families, as has been previously observed, there was evidence for genetic heterogeneity between the fragile X locus and the F9 site. The observed recombination frequencies were as follows: DXS51-F9, 0%; F9-DXS52, 45%; DXS51-FRAXA, 15%; F9-FRAXA, 18%; DXS98-FRAXA, 36%; DXS52-FRAXA, 15%. The authors proposed the following relative order for the 5 loci, based on multipoint linkage analysis: (DXS51, F9, DXS98)--FRAXA--DXS52. |
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