Inheritance of an Illness
National Center for Human Genome Research, National Institutes of Health. "New Tools for Tomorrow's Health Research." Bethesda, MD: Department of Health and Human Services, 1992.
People like most other animals and plants, have two copies of every chromosome and, therefore, two copies of every gene. However, the two members of any gene pair - one inherited from the mother and the other from the father - do not necessarily have identical DNA sequences. If a person carries two different DNA sequences at a particular site on a chromosome, the person is said to be heterozygous at that site. If they are both the same, the person is homozygous at the chromosome site.
Whether a person is homozygous or heterozygous for a given gene can sometimes mean the difference between health and disease. A well-understood example of this is the inherited disease sickle cell anemia. Sickle cell disease is due to defective hemoglobin, the protein that fills red blood cells and carries oxygen. The defective protein is caused by a defect in one gene. Moreover, this defective gene must be present in two copies before the disease arises. People with only one copy of the defective gene - who are heterozygous - are usually healthy and are called "carriers."
In the United States, about 9 percent of blacks are heterozygous for sickle cell trait, while much fewer than 1 percent are homozygous and show symptoms of the disease. In some African populations, almost 45 percent of the people carry one gene for the sickle cell trait.
Scientists believe that the heterozygous condition confers some advantage to those who carry it. Scientists now know that people who are heterozygous for the sickle cell trait are more resistant to infection by the organism that causes malaria. Inhabitants of areas in Africa where malaria is most prevalent are very often carriers of the sickle cell trait. Moreover, women carriers seem to be more fertile than women who have two normal genes.
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