Every living cell contains genes. They are too small to be seen in a microscope, but they are very 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 . For example, something might be wrong with the gene that makes blood clot . A baby born with this defective gene could suffer serious hemorrhages  or even bleed to death because his or her blood fails to clot. If scientists ever learn how to map all the genes in our bodies, they can determine whether or not an unborn child has any defective genes. They might even discover how to treat these genes before the child is born. What is the main idea of this passage?
Defective genes can never be repaired. Genes are too small to be seen through a microscope. Genes help scientists understand how living things develop their characteristics. The Human Genome Project may explain the role of every gene in the body.