The genes that help determine a person's individual characteristics—from the color of his

eyes to the score he makes on an IQ test—are located on chromosomes within the cells of his body. Half of a person's chromosomes come from his father, half from his mother. Many diseases are the result of a single defective gene on one of the chromosomes. Achondroplastic dwarfism, for example, is caused by a dominant gene and any child who inherits it will have the disease. A genetic counselor confronted by a parent with such a disease could warn that half of his children risk the disease. More often, genetic diseases are caused by recessive genes. The most common is cystic fibrosis, a disorder that affects at least one in every 1600 babies and causes their lungs and other body organs to become congested with mucus. Some diseases, such as the blood-clotting disorder, hemophilia, are sex-linked recessive defects carried on the female X chromosome.

Many genetic defects, such as the hemophilia of European royalty, can be traced back through the family tree. And a genetic counselor can calculate probable risks for couples even before they have defective children. But faulty genes may also occur without warning by mutation in any generation.

In recent years, researchers have detected a number of disorders caused by an extra chromosome, or lack of part of a chromosome. Mongolism, a form. of retardation accompanied by short stature, a flattened nose and broad hands and feet, is caused by an extra chromosome. The parents of such a child have little increased risk of having another Mongoloid.

There are blood, urine and other tests which show promise in detecting more than 100 genetic diseases, including cystic fibrosis, hemophilia and some forms of muscular dystrophy.

Researchers are also detecting genetic defects even before a child is born. "Intrauterine detection," notes Dr. Henry L. Nadler of Northwestern University Medical School, "brings a new dimension to genetic counseling. The physician may now inform. the parents that they will have either an affected or a normal child. "

The original title of the passage can be best replaced by______.

A．Genes and Diseases

B．Genes and One's Characteristics

C．Genetic Defects and the Family Tree

D．Dominant and Recessive Genes