Every living cell(细胞) contains genes(基因). They are too small to be seen under a microscope, but they are vitally important. Each set of genes in the body contains all the instructions needed to make a human being. Some genes determine hair color. Some determine the shape of a nose. Some genes help determine your height and even your weight.

　　Genes are made of a chemical called DNA — the letters stand for deoxyribonucleic acid. In the early 1950’s two scientists, Francis Crick and James Watson, figured out how the parts of DNA fit together. Once scientists understood this structure, it became possible to take pieces of DNA apart and put them together in new ways. New kinds of genes could be made in this manner.

　　Scientists have studied the genes of many plants and animals. They have worked out which genes affect the color of a tomato and the thickness of its skin. Working out which genes determine which features is called genetic mapping, scientists have begun the Human Genome Project, an ambitious effort to map all the genes in the human body.

　　Some genes may be defective(有缺陷的). For example, something might be wrong with the gene that makes blood clot(凝聚). An individual born with this defective gene could suffer serious hemorrhages(出血) or even bleed to death because his or her blood fails to clot. Scientists are trying to treat the defective genes, and have made great achievements. If scientists could know a child has any defective genes beforehand, they might even discover how to treat these genes before the child is born.

　　1. What was the achievement of francis Crick and James Watson?

　　A. They discovered why people have different hair colors. B. They learned that some people bleed for long periods of time.

　　C. They looked at human cells under the microscope. D. They worked out the structure of DNA.

　　2. What is the main idea of this passage?

　　A. Defective genes can never be repaired. B. Genes are too small to be seen through a microscope.

　　C. Genes help scientists understand how living things develop their characteristics.

　　D. The Human Genome Project may explain the role of every gene in the body.

　　3. Which of the following statements is a fact?

　　A. Genes help determine your height and weight.

　　B. Genetic engineering is the most important scientific discovery of the last 25 years.

　　C. It is dangerous to make changes in genes. D. Supermarkets should not sell genetically engineered food.

　　【答案与解析】 基因是由叫做DNA的化学物质构成的。科学家们已经开始绘制人体基因图了。

　　1. D。细节题。命题依据是第 2 段第 2 句话In the early 1950’s two scientists, Francis Crick and James Watson, figured out how the parts of DNA fit together。

　　2. C。主旨题。从文章内容可知，不同的基因有不同的功能。所以，本文主旨应该是C项。

　　3. A。判断题。只有A项内容文章涉及了，B、C、D三项内容文章没有体现。