noun
also fragile x
Usage: usually capitalized x
Etymology: so called from a fragile (partially gapped) site at band Xq 27 of the X chromosome in those affected with the disorder
: an X-linked inherited disorder that is caused by a genetic mutation marked by elongation of a normally occurring tandemly repeated trinucleotide sequence, that is characterized by moderate to severe mental retardation, by large ears, chin, and forehead, and by enlarged testes in males, and that often has limited or no effect in heterozygous females