46714.2 Spontaneous mutationto 1 of 1500 males and 1 of 2500 females. It is mani-fested cytologically by a fragile site in the X chromo-some that results in breaks in vitro. Fragile X syndromeresults from changes in the number of a (CGG)nrepeatin a region of the FMR-1gene that is transcribed but nottranslated (Figure 14-24a).How does repeat number correlate with the diseasephenotype? Humans normally show considerable varia-tion in the number of CGG repeats in the FMR-1gene,ranging from 6 to 54, with the most frequent allele con-taining 29 repeats. Sometimes, unaffected parents andgrandparents give rise to several offspring with fragile Xsyndrome. The offspring with the symptoms of the dis-ease have enormous repeat numbers, ranging from 200 to1300 (see Figure 14-24b). The unaffected parents andgrandparents have also been found to contain increasedcopy numbers of the repeat, but ranging from only 50 to200. For this reason, these ancestors have been said tocarry premutations.The repeats in these premutation al-
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.