问题 选择题

关于人类性别决定的叙述正确的是(  )

A.人类的性别决定方式是XY型

B.生男生女主要取决于母亲,与父亲无关

C.若第一胎生了个女儿则第二胎生儿子的可能性为100%

D.俗话说“酸儿辣女”说明生男生女由饮食决定

答案

男性体细胞中的染色体是22对常染色体+XY,女性体细胞的染色体是22对常染色体+XX.

在亲代的生殖细胞形成过程中,经过减数分裂,染色体彼此分离,男性产生两种类型的精子--含22+X染色体的精子和含22+Y染色体的精子.女性则只产一种含22+X染色体的卵细胞.受精时,如果是含22+X的精子与卵子结合,就产生具有44+XX的受精卵并发育成女性;如果是含22+Y的精子与卵子结合,就产生具有44+XY的受精卵并发育成为男性.可见,生男生女主要决定于父亲的哪一种精子与卵细胞相融合.由于男性可产生数量相等的X精子与Y精子,加之它们与卵子结合的机会相等,所以每次生男生女的概率是相等的.在整个人群中男女性别之比大致1:1.因此若第一胎生了个女儿则第二胎生儿子的可能性为50%.

故选A

阅读理解

It was a beautiful Sunday morning, and Maggie and I were returning from our walk through the woods.We were only a couple of blocks from home when I spotted a cell phone and a credit card sitting on the road.We took them home.We always find amazing things on the street and she looks upon them as a movable feast-a chicken wing here or a barbecue rib (肋骨)there.

I found another cell phone a few years back, too, and called a number in its phone book.I explained the situation to the guy who answered.He said it was his sister’s and that he’d come to pick it up, which he did.

And that was that.No verbal thank-you, no written thank-you, no “here’s a box of chocolates” thank-you.

I didn’t have time to call anyone on my latest found cell phone.I was pouring myself coffee when it started to shake and dance across the kitchen counter.

“Who’s this?” someone asked when I picked up.

“Who’s this?” I countered(反问).“Sarah?”

She was surprised at my knowing her name until she realized her name was on the credit card.“Could you send them to me?” she asked.

She lives in Arlington, which is 2 miles from my house.

“Hmm, no, ”I replied, adding that I thought she could come to get them, and that if I wasn’t at home, they would be in my mailbox.

A day later, when I was out for a run, someone got them back.There wasn’t even a piece of paper put in the mailbox with “Thanks” on it.In this age of e-mail and cell phone, there’s really no excuse.Years ago, I found something more precious than a $100 bill on the street:a driver’s license.I saw  that its owner lived a couple of blocks from me, so I called him up.He asked whether I could slip the license through his front door.

“I guess I could, ”I replied.

And that was that.

小题1:What is the relationship between Maggie and the writer?

A.Wife and husband

B.Daughter and father

C.Teacher and student

D.Master and pet dog小题2:.How many experiences are mentioned by the writer to return things to the losers?

A.3

B.4

C.5

D.6小题3:How did the writer know Sarah’s telephone number?

A.From her telephone’s phone book

B.From her credit card

C.From her e-mail

D.From her driver’s license小题4:The writer wants to tell us through the unusual stories,

A.we should return the things we picked to the losers

B.people don’t know how to appreciate others in the age of e-mail and cell phone

C.people would learn to appreciate persons who provide help for you

D.the advance of society makes people lose some virtues

单项选择题