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Genetic heterogeneity of X‐linked mental retardation with fragile X Association of tight linkage to factor IX and incomplete penetrance in males

X‐linked mental retardation with fragile X or Martin–Bell Syndrome (MBS) is a frequent cause of mental retardation. So far segregation analysis of MBS in pedigrees ascertained by different, incomplete criteria has produced results, difficult to interpret, which suggest genetic complexity (Sherman et al. 1985). Biochemical and cell biological studies have failed to provide an assay for genetic heterogeneity in MBS and linkage analysis is the only available method. Such analysis, however, is complicated by the incomplete penetrance of the disease in males and the variable penetrance and expression of the defect in heterozygous females.