Introduction to Genetic Analysis 468

Introduction to Genetic Analysis 468 - 44200_14_p451-480...

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467 14.2 Spontaneous mutation to 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 syndrome results from changes in the number of a (CGG) n repeat in a region of the FMR-1 gene that is transcribed but not translated (Figure 14-24a). How does repeat number correlate with the disease phenotype? Humans normally show considerable varia- tion in the number of CGG repeats in the FMR-1 gene, ranging from 6 to 54, with the most frequent allele con- taining 29 repeats. Sometimes, unaffected parents and grandparents give rise to several offspring with fragile X syndrome. The offspring with the symptoms of the dis- ease have enormous repeat numbers, ranging from 200 to 1300 (see Figure 14-24b). The unaffected parents and grandparents have also been found to contain increased copy numbers of the repeat, but ranging from only 50 to 200. For this reason, these ancestors have been said to carry 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.

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