问题 选择题

从染色体组成看,正常男性产生的精子类型及其染色体组成分别是(  )

A.两种  22对+X或Y

B.一种   22对+XY

C.两种  22条+X和Y

D.两种   22条+X或Y

答案

每个正常人的体细胞中都有23对染色体,其中有22对男女都一样,叫常染色体,有1对染色体,因男女而有差异,叫性染色体,男性为XY,女性为XX.体细胞中染色体是成对存在,在形成精子和卵细胞的细胞分裂过程中,染色体都要减少一半.而且不是任意的一半,是每对染色体中各有一条进入精子和卵细胞.生殖细胞中的染色体数是体细胞中的一半,成单存在.女性只产生一种含X染色体的卵细胞,染色体组成是:22条+X,男性精子的性染色体有两种,一种是含X染色体的,一种是含Y染色体的,染色体组成是:22条+X 或22条+Y.因此,选项D符合题意.

故选:D

阅读理解

阅读理解

     Man, a land animal. But he is also closely tied to the sea. Throughout history the sea has served the

needs of man. The sea has provided man with food and a convenient way to travel to many parts of the

world. Today, nearly two-thirds of the world's population lives within 80 kilometers of the sea coast.

    In the modern technological world, the sea offers many resources to help mankind survive. Resources

on land are beginning to be used up. The sea, however, still can be hoped to supply many of man's needs.

    The list of riches of the sea yet to be developed by man's technology is big. Oil and gas explorations

have been carried out for nearly 30 years. Valuable amounts of minerals exist on the ocean floor ready

to be mined.

    Fish farming promises to be a good way to produce large quantities of food. The culture of fish and

shellfish (贝类动物) is an ancient skill practised in the past mainly by Oriental people, including people

in China.

    Besides oil and gas, the sea may offer new sources of energy. Experts believe that the warm

temperature of the ocean can be used in a way similar to the steam in a steamship. Ocean currents and

waves offer possible use as a source of energy.

    Technology is enabling man to explore ever more deeply under the sea. The development of strong,

new materials has made this possible.

The technology to harvest the sea continues to improve. Experts believe that by the year 2020 the

problems that prevent us from exploiting (开发) fully the food, minerals, and energy sources of the sea

will be largely solved.

1. The major things that the sea offers man are _________.  

A. fish and oil                          

B. minerals and oil

C. warm temperature and ocean currents      

D. the food, energy sources, and minerals

2. We can conclude from this passage that _________.  

A. the sea resources have largely been used up

B. the sea, in the broad sense, has not yet been developed 

C. the problems preventing our using the resources of the sea have already been solved.

D. by the year 2020, the technology will be good enough to exploit all the sea resources.

3. The underlined words "Oriental people" in the fourth paragraph probably mean _________.

A. the people in Asia

B. African people

C. European people

D. American people

4. The best title for this passage is _________. 

A. Sea Food      

B. Technology for Exploiting the Sea

C. Sea Harvest    

D. Man, nature and the Sea

单项选择题