How
do genes work?
- Even though every cell contains a full
set of DNA, genes are only used by cell selectively. Housekeeping
genes are genes that enable cells to make proteins required for
basic functions, while other genes are inactive for most of the
time. Scores of genes encode proteins that are exclusive to a
certain type of cell and that give the cell its. For example,
proteins required in a liver cell are very different from those
in a bone cell. A normal cell activates just the genes it needs
at the moment and actively suppresses the rest. Genes determine
all body processes, including the body's reactions to the environment
through the proteins that they encode.
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Inheritance
of an Illness
 Everyone
has two copies of every chromosome and, hence, two copies of every
gene. Nevertheless, the two members of any gene pair - one inherited
from the father and the other from the mother -may not necessarily
have identical DNA sequences. A person is said to be homozygous
at a chromosome site if both the DNA sequences are the same. The
person is said to be heterozygous at a site of a chromosome if
he carries two different DNA sequences at that site. Being homozygous
or heterozygous for a given gene can sometimes mean the difference
between health and disease. - An example of this is the inherited disease
is sickle cell anemia. Sickle cell disease is caused by malfunctioning
hemoglobin, the protein that carries oxygen in red blood cells.
The faulty protein is caused by a defect in one gene. Likewise,
this defective gene must be present in two copies before the disease
develops. Anyone with only one copy of the defective gene (heterozygous)
is usually healthy and are called "carriers." Scientists believe
that the heterozygous condition confers some advantage to those
who carry it. Scientists have discovered that people who are heterozygous
for the sickle cell trait are more resistant to infection by the
organism that causes malaria. Carriers of the sickle cell trait
are often inhabitants of areas in Africa where malaria is most
common. Furthermore, women carriers seem to be more fertile than
women who have two normal genes.
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