The fragile X prevalence paradox

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Abstract
A second difficulty with population studies is the tendency to conflate disease prevalence (projections from SEN cohorts) with allele/carrier frequencies within the general population, as prevalence estimates will only approach carrier frequencies for complete penetrance. Thus, seemingly paradoxical results among studies of prevalence may reflect the effects of selection bias, conflation and differing defining criteria for FXS across studies. Aspects of this “paradox” are exemplified by two important screening studies of Israeli women. Using the same cut-off point (55 CGG repeats) for premutation (PM) carriers, Toledano-Alhadef et al8 found a higher frequency of carriers (127/14 334 = 1/113) than did Pesso et al9 (62/9459 = 1/152), despite the fact that Pesso et al reported a higher frequency for full mutation (FM) alleles (4 FM carriers; 1/2365) than did Toledano-Alhadef et al (3 FM carriers; 1/4778). Although the numbers of FM alleles in the two studies are too small to attach significance to the difference in frequencies, the trend is consistent with the exclusion of individuals with a family history of …