Scientists have long suspected that such genetic benefits might accrue from having an extra chromosome 21.
科学家更深入地推测，这些基因上的益处也许能在那条额外的21号染色体上获得增加。
Men may have an extra X chromosome or two on top of their normal XY combination.
男人可能有除了他们的正常的XY组合以外还有额外的1个或2个X染色体。
Down syndrome (trisomy 21) is a disorder caused by the presence of an extra 21st chromosome.
唐氏综合症（21三体综合症）是由于患者额外多了一条21号染色体所致的疾病。
Often, the error resulted in an extra copy of chromosome 21, which causes Down's syndrome.
通常情况下，错误导致21号染色体的多余复制，引发唐氏综合症。
Often, the error resulted in an extra copy of chromosome 21, which causes Down's syndrome.
通常情况下，错误导致21号染色体的多余复制，引发唐氏综合症。
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