问题 问答题

在正常情况下,只有一个精子与卵细胞结合形成受精卵,在特殊情况下,两个卵细胞同时分别与两个精于结合,即两个卵细胞同时受精,这种情况下,就会同时形成两个胚胎,即双胞胎,那么这对双胞胎的性别有几种可能性?

答案

如果两个卵细胞都与含有X染色体的精子结合,子代的性别都为女性;如果两个卵细胞都与含Y染色体的精子结合,子代的性别都为男性;如果两个卵细胞分别与含有X染色体和含有Y染色体的精子结合,子代的性别将是一男一女。  因此,双胞胎的性别有三种可能。

阅读理解

阅读理解。

     Top lists are lecturing people on everything from"100 books to read ".

     Aren't you just tired of being told what to do with your time?

     Now you have a list to end all lists!

     Take a look at the following two examples from the list of "101 things not to do":

     Swim with Dolphins (海豚)?

     Swimming with dolphins is one of the world's most profitable tourist activities. However, every

dolphin will welcome having their busy ,tiring day interrupted by tourists screaming pushing around them

in the water. Worse yet, when dolphins get too near to the boats loaded with tourists, they could get

caught up in ropes and killed by propellers(螺旋桨).

     Here's a little secret. Dolphins look like smiling at you, but actually they're just opening mouths.

     Here's a little secret. Dolphins look like smilingat you, but actually ther're just opening their mouths.

     Go to See the Mona Lisa?

    There must be something about the mysterious (神秘的) smile. The 6 million people who visit the lady

in the Louvre every year can't all be wrong, after all. But they can be quite annoying, standing in front of

you, holding up their cameras to prevent you from seeing anything. In fact,it is hard for you to see the

painting clearly because you have to stay away from it for security reasons. After queuing for hours, many

tourists can remain in front of the painting only for 15 seconds at most.

     If the mysterious lady in the picture knew her fate, she wouldn't just be smiling, she'd laughing.

So, still long to see the Mona Lisa? If you want to find out more about the list, read 101 Tings NOT to

Do Before You Die. Visit www.not2dobeforeidie.co.uk and buy the book at a 20% discount.

1. According to the passage,swimming with dolphins________.

A. is the world's most popular tourist activity

B. gives fun to both tourists and dolphins

C. will make tourists busy and tired

D. can cause danger to dolphins

2.what does the author think about going to see the Mona Lisa?

A. Tt is wrong to go and see the mysterious smile.

B. It is not as satisfying as expected.

C. Fifteen seconds in front of the painting is enough.

D. Queuing for hours is worthwhile.

3.The list of "101 things not to do"is made most probably because its author_______.

A.thinks it boring to do the things suggested by other lists

B.believes other lists are not humorous enough

C.intends to persuade people to read more lists

D.wants to provide a list different from other lists

43.What is the main purpose of the passage? 

A.To advertise a book

B.To introduce a website 

C.To comment on popular lists

D.To recommend tourist activities.

单项选择题