In sex-linked inheritance, the gene responsible for the disease is located on the X chromosome. Usually, the abnormal gene is recessive. For these reasons, the resultant disorder is called an X-linked recessive disease. In a woman with such a defective gene, the effects of the abnormal gene are masked by those of the normal gene on the other X chromosome. Although she does not have the disease herself, she is a carrier, capable of transmitting the defective gene to her children.
In X-linked recessive disease, the Y chromosome lacks the corresponding normal gene to mask the harmful effects of the abnormal gene on the X chromosome. Thus, all male offspring of a woman who is a carrier of an X-linked recessive disease have a 50 percent chance of having the condition. All female offspring have a 50 percent chance of being carriers. Following are examples of X-linked recessive disorders:
Color blindness: A person with the most common form of color blindness cannot distinguish red from green hues.
Hemophilia: In hemophilia, the blood does not clot properly. Persons with hemophilia bleed excessively, even from minor cuts. There are several forms of hemophilia, each caused by a deficiency of a different protein called a clotting factor. The disease is managed by giving transfusions of the deficient clotting factor and of whole blood to replace blood losses. Research continues toward the accurate identification of carriers and prenatal diagnosis of this disease.
Duchenne muscular dystrophy: Muscular dystrophy refers to a group of rare diseases characterized by progressive muscular weakness. Duchenne type muscular dystrophy is the most common. Between the ages of two and six years of age, children with this condition develop weakness first in their legs, then in their arms and trunk. The weakness rapidly worsens. Most children die during their second decade, usually as a result of severe weakness of the muscles of respiration. The disease has no cure. Treatment includes physical therapy, braces, and, occasionally, surgery.
Newer techniques and better understanding of the genes involved have made prenatal testing possible. Advances in genetic testing are soon likely to make it possible to detect the recessive trait in carriers as well.