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X-连锁智力低下综合征和巨睾症与X染色体长臂远端2区7带fra(Xq27)脆性位点有关,在无叶酸的培养基培养中期淋巴细胞可以显示脆性X-染色体,这是遗传决定的智力发育障碍中最常见的一种形式。临床上对于大部分是正常的杂合子的检出和妊娠时半合子的检出是可能的。但是,这里的主要困难是专性杂合子不容易被发现,而且从半合子成纤维细胞培养证明脆性部位是非常困难的,在中期淋巴细胞大约有6～60%表现这种特征,在成纤维细胞也只有一部分细胞中有脆性X 染色体。
X-linked mental retardation syndrome and macroscopoeia are associated with fra (Xq27) fragile sites in the distal 7 region of the long arm of the X chromosome, and metaphase lymphocytes can develop fragile X-chromosomes in culture without folic acid The most common form of genetically determined mental retardation. It is clinically possible to detect the majority of normal heterozygotes and detect hemizygotes during pregnancy. However, the main difficulty here is that obligate heterozygotes are not easily detected and it is very difficult to prove the fragility site from hemizygous fibroblasts. Approximately 6 to 60% of mid-term lymphocytes exhibit this characteristic. In fibroblasts Only a part of the cells have fragile X chromosomes.