RNA干涉（RNAi）
参考答案：
转录的外源基因同时抑制自身，其内源基因表达的基因沉默现象。
When asked to explain ________ made his lessons so exciting, the teacher kept silent.
A．what it was that
B．that what it was
C．what was it that
D．that what was it
21-三体综合征属于（）
A．染色体病
B．单基因遗传病
C．多基因遗传病
D．体细胞遗传病
E．线粒体病
