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