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Mental retardation is more frequent in boys than girls, a finding attributed in recent decades to mutations in X-linked genes.1 Analysis of X-chromosome mutations has enriched our understanding of human disease. The variability of phenotypes associated with mutations in genes on the X-chromosome relates to a number of factors including the extent of expression in hemizygotes (females with Turner syndrome and males) who have only one X-chromosome vs heterozygotes (females with two X-chromosomes). Lyonization, the random inactivation of one of the two X-chromosomes, ensures balanced gene dosage, and compensates for mutations in one allele when the process is skewed in favor of the wild-type allele.2,3⇓ As is the case for genes at any locus, the nature of the alteration in the DNA sequence determines the effect on the gene product and its function. The individual’s genetic background and environment may further modify the phenotype.
The discovery that mutations in the FMR1gene cause fragile-X A (FRAXA) syndrome marked the beginning of an explosion of knowledge concerning X-linked mental retardation (XLMR). The finding that FRAXA is caused by expansion of a CGG repeat …
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