用所给词的适当形式填空。

1. _____ ( eight, year, old) should be allowed to drive.

2. Anna wants to get her ears_____ ( pierce).

3. Young trees should            _____( water) as often as possible.

4. You need             ______ (wear) warm clothes, or you'll catch a cold.

5. The old should                   __ (speak) to politely.

简述遗传病的类型。