Scientists have long suspected that such genetic benefits might accrue from having an extrachromosome 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。
2
I owe my ability to accept maternal failure to my son Adam. Though I bred young, never smoked or drank, ate right, and all that, Adam showed up with an extrachromosome, mentally retarded.