问题 问答题 简答题

什么是受精卵的性别决定?

答案

参考答案:

生殖细胞在形成过程中经过减数分裂,同源染色体彼此分开。故精子和卵子各有半数的同源染色体,称为单倍体(23条)。受精卵形成后,染色体又复原成二倍体(46条)。生殖细胞的染色体在世代传递中保持稳定的数目,得以维持其遗传性。

卵子的染色体核型只有一种:22+X;而精子的核型有两种:X型精子(22十X)与Y型精子(22+Y)。若X型精子与卵子结合,则形成XX型受精卵,发育成女性;若Y型精子与卵子结合,则形成XY型受精卵,发育成男性。生殖细胞染色体决定受精卵的性别分化,称为性别决定。

阅读理解

阅读理解。

Dear students,

     I'm very happy to be your guest speaker today. It's a great pleasure to visit your school and meet all of

you here.

     I would like to tell you about myself. I have been a reporter for the last fifteen years. I chose this job so

I could travel the world, but the job has taught me many unforgettable lessons. My work is sometimes boring

but for most of the time it is amazing and inspiring. I have seen wars, earthquakes,poverty and death. But I

have also seen courage, hope and happiness.

     In India, I visited a city. There were many homeless children in it. Some were young and lived in the streets.

But then a wonderful lady called Rosa opened a home for them. In one whole year, she was looking after two

hundred kids, feeding them, teaching them anti giving them hope.

     Another time, I was in Japan and met a young girl with a serious illness after seven operations (手术), she

had to have thirteen more in hospital. However, when I met her, she was still smiling.

     Here in China after the terrible Wenchuan Earthquake, I found a three-year-old boy whose name is Lang

Zheng. He had been in the ruins (废墟) for more than ten hours before he was found and saved out by the

rescuing soldiers. Though weak and injured, the brave boy lifted his right hand above the head with difficulty

and saluted (向…致敬) the soldiers around, which became the most moving moment in the disaster (震灾).

     Thanks to the experience as a reporter, I myself am getting braver and stronger. When I face challenges in

my life. I always remember the courage of the three people.

1. Why did the speaker choose the job of being a reporter?

A. Because he tried to improve his speaking skills.

B. Because he wanted to experience difficulties.

C. Because he dreamed of traveling the world.

D. Because he was willing to offer help to others.

2. From the passage,We know that Rosa was _____.

A. kind and patient

B. creative and proud

C. pushy and lucky

D. modest and helpful

3. The Japanese girl had to have _____ operations in all (总共).

A. 7

B. 13

C. more than l3

D. 20

4. Little Lang Zheng lifted his right hand in order to _____.

A. tell people around he was still alive

B. show thanks to the soldiers

C. prove he was special

D. have some photos taken for him

5. The passage above is _____.

A. a news report

B. a letter

C. a speech

D. a documentary

单项选择题 B1型题