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问题 解答题

如图是生物生殖过程中染色体变化示意图，据图回答：

（1）请画出图中精子、卵细胞、受精卵细胞中染色体和基因；

（2）若图为人类生殖过程中染色体变化的示意图，图中所示的是一对性染色体，父方体细胞中性染色体组成为XY，则产生的精子有两种，分别为含______和含______性染色体，母方体细胞中的性染色体组成为______，产生的卵细胞中只含______性染色体；

（3）从生殖细胞结合形成受精卵的角度看，决定生男生女的关键是由______生殖细胞所决定；从理论上讲，男女比例大致是______．

答案

（1）在亲代的生殖细胞形成过程中，经过减数分裂，成对的染色体和基因分开，分别进入两个生殖细胞中．父亲产生的精子中含有一条具有基因A的染色体，母亲产生的卵细胞中含有一条具有具有a的染色体；受精卵中的染色体一条来自父方，一条来自母方，这样受精卵中的染色体数又恢复到亲代体细胞中的染色体数，保证了亲子代间遗传物质的稳定性．因此受精卵中含有父方提供的一条染色体和母方提供的一条染色体，基因组成是Aa．

（2）男性的性染色体为XY，女性的性染色体为XX．在亲代的生殖细胞形成过程中，经过减数分裂，两条性染色体彼此分离，男性产生两种类型的精子--含X染色体的精子和含Y染色体的精子．女性则只产一种含X染色体的卵细胞．受精时，如果是含X的精子与卵子结合，就产生具有XX的受精卵并发育成女性；如果是含Y的精子与卵子结合，就产生具有XY的受精卵并发育成为男性．

（3）从生殖细胞结合形成受精卵的角度看，受精时，精子和卵细胞的结合是随机的，可见后代的性别是由父（男）方产生的生殖细胞（精子）类型所决定的．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．

故答案为：（1）；

（2）X；Y；XX；X

（3）精子；1：1．

单项选择题

On a weekday night this January, thousands of flag-waving youths packed Olaya Street, Riyadh’s main shopping strip, to cheer a memorable Saudi victory in the GCC Cup football final. One car, rock music blaring from its stereo, squealed to a stop, blocking an intersection. The passengers leapt out, clambered on to the roof and danced wildly in front of the honking crowd. Having paralyzed the traffic across half the city, they sped off before the police could catch them.
Such public occasion was once unthinkable in the rigid conformist kingdom, but now young people there and in other Gulf states are increasingly willing to challenge authority. That does not make them rebels: respect for elders, for religious duty and for maintaining family bonds remain pre-eminent values, and premarital sex is generally out of the question. Yet demography is beginning to put pressure on ultra-conservative norms.
After all, 60% of the Gulf’s native population is under the age of 25. With many more of its citizens in school than in the workforce, the region faces at least a generation of rocketing demand for employment. In every single GCC country the native workforce will double by 2020. In Saudi Arabia it will grow from 3.3m now to over 8m. The task of managing this surge would be daunting enough for any society, but is particularly forbidding in this region, for several reasons.
The first is that the Gulf suffers from a lopsided labor structure. This goes back to the 1970s, when ballooning oil incomes allowed governments to import millions of foreign workers and to dispense cozy jobs to the locals. The result is a two-tier workforce, with outsiders working mostly in the private sector and natives monopolizing the state bureaucracy. Private firms are as productive as any. But within the government, claims one study, workers are worth only a quarter of what they get paid.
Similarly, in the education sector, 30 years spent keeping pace with soaring student numbers has taken a heavy toll on standards. The Saudi school system, for instance, today has to cope with 5m students, eight times more than in 1970. And many Gulf countries adapted their curricula from Egyptian models that are now thoroughly discredited. They continue to favor rote learning of "facts" intended to instill patriotism or religious values.
Even worse, the system as a whole discourages intellectual curiosity. It channels students into acquiring prestige degrees rather than gaining marketable skills. Of the 120, 000 graduates that Saudi universities produced between 1995 and 1999, only 10,000 had studied technical subjects such as architecture or engineering. They accounted for only 2% of the total number of Saudis entering the job market.

According to the author, the Saudi education system is characterized by its excessive emphasis on

A．technical creativity.

B．intellectual cognition.

C．conservative values.

D．nonconformist images.
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单项选择题

车孔刀可分为通孔车刀和盲孔车刀两种，通孔车刀的几何形状与外圆车刀基本上相似，主偏角一般在（）。

A.60～75°

B.90°

C.大于90°

D.15°～30°
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