题目

某遗传病是由一对等位基因控制的。小明和他的父亲得了这种遗传病。经测定，他的母亲不含有该致病基因。推测该病的致病基因可能是( ) A．X染色体上的隐性基因 B．X染色体上的显性基因 C．常染色体上的隐性基因 D．常染色体上的显性基因   答案：D用排除法。因为母亲是纯合子，儿子患病，致病基因不可能是隐性基因，排除A、C两项。若为X染色体显性遗传，儿子患病，其母亲必患病，与题干不符，排除B项。故该病的致病基因可能是常染色体上的显性基因。  V.用所给词的适当形式完成短文Last week,Tony 1. (have) a very busy weekend. On Saturday morning,he2. (clean) his room. In the afternoon,he3. (do) his homework. It 4.         (be) very easy,so it 5. (take) him only half an hour. In the evening,he 6. (visit) his friends and they 7. (have) a good supper. On Sunday morning,he 8. (get) up very late. Then he 9. (read) a book about Chinese history. In the afternoon,he 10.        (go) to see an interesting movie.