$\frac{\text{1}}{\text{6}}÷\frac{\text{4}}{\text{9}}÷\frac{\text{1}}{\text{3}}$

填上合适的词语。

一____高山  一____深涧   一____野果 

一____泉水   一____血泡  一____巨响

    My name is Tom Brown. I am an English boy. (1) I am twelve. I am a middle school student. From Monday to Friday I am very busy. We have six classes a day. At school, I like doing sports, (2) like playing basketball, tennis and soccer. So my favorite subject is P.E.. Mr. White is my P.E. teacher. He is very cool. After school, our P.E. teacher plays games with us. We students all like him. (3) I don't like math because it is too difficult for me. (4) But my math teacher Ms Smith says it is interesting and useful. So I must work hard on it.

视神经萎缩症是一种显性遗传病，若一对夫妇均为杂合子，生正常孩子的概率为：