Download Name______________________ The Sickle Cell Gene Background

Name______________________
The Sickle Cell Gene
Background
Sickle cell disease (also called sickle cell anemia) is an inherited blood disorder
that affects red blood cells. The sickle cell gene causes the body to produce
abnormal hemoglobin. In sickle cell disease, the hemoglobin clumps together,
which causes red blood cells to become stiff and develop a C-shaped (“sickle”)
form. These sickled red blood cells can block blood vessels, reducing blood flow
in many parts of the body. This process results in tissue and organ damage.
Risk Factors
Sickle cell disease is inherited. People at risk for inheriting the gene for sickle cell
descend from people who are or were originally from Africa and parts of India
and the Mediterranean. The sickle cell gene also occurs in people from South and
Central America, the Caribbean, and the Middle East. The high occurrence of the
sickle cell gene in these regions of the world is due to the sickle cell's ability to
make red blood cells resistant to the malaria parasite:
People who inherit just a single gene are referred to as having the
sickle trait. These people are protected against malaria and do not develop
sickle cell disease. About 40% of people in certain parts of Africa and
about 9% of African-Americans have the trait.
Those who inherit both copies of the HbS gene develop sickle cell
disease. They are not protected from malaria, however. In fact, malaria is
more serious in these individuals. An estimated 1 in 500 AfricanAmericans and 1 in 1,000 - 1,400 Hispanic Americans are born with sickle
cell disease.
RISK OF INHERITANCE
The sickle cell gene for hemoglobin S (HbS) is the most common inherited blood
condition in the United States. About 70,000 - 100,000 Americans -- mostly
African-Americans -- have sickle cell disease. About 2 million Americans have
sickle cell trait.
People inherit a pair of genes that regulate hemoglobin, with one gene coming
from each parent. If two sickle genes are inherited, a person will have sickle cell
disease. If a one normal hemoglobin gene and one sickle cell gene are inherited, a
person will have sickle cell trait. People who have sickle cell trait are healthy and
do not develop themselves sickle cell disease, but they are “carriers” who can pass
the disease on to their children.
The risk of a child inheriting sickle cell disease or sickle cell trait is as follows:
If both parents have sickle cell trait (each have one normal
hemoglobin gene and one sickle cell gene), the child has a 50% chance of
inheriting sickle cell trait (one normal gene, one sickle cell gene), 25%
chance of inheriting sickle cell disease (two sickle cell genes), and 25%
chance of not inheriting either the trait or the disease (two normal genes).
If one parent has sickle cell trait (one normal gene and one sickle
cell gene) and the other parent has two normal hemoglobin genes, the
child has a 50% chance of inheriting sickle cell trait (one normal gene and
one sickle cell gene) and a 50% of inheriting neither the trait nor the
disease (two normal genes). The child is not at risk of inheriting sickle cell
disease.
If one parent has sickle cell disease (two sickle cell genes) and the
other parent has sickle cell trait (one normal gene, one sickle cell gene),
the child has a 50% chance of inheriting sickle cell trait and a 50% chance
of inheriting sickle cell disease.
If one parent has sickle cell disease and the other parent has two
normal hemoglobin genes, the child has a 100% chance of inheriting sickle
cell trait, but not the disease.
If both parents have sickle cell disease, the child has a 100% chance
of inheriting the disease.
Notes:





According to the latest estimates, there were about 219 million cases
of malaria in 2010 (with an uncertainty range of 154 million to 289
million) and an estimated 660 000 deaths (with an uncertainty range
of 490 000 to 836 000).
Forty percent of the world population is at risk for malaria.
Ten percent of world population gets sick each year with malaria.
In equatorial Africa, up to 40% of people are carriers of this mutated
gene.
People who carry the sickle cell trait have some resistance to malaria.
Their red blood cells, containing some abnormal hemoglobin, tend to
sickle when they are infected by the malaria parasite. Those infected
cells flow through the spleen, which kills them because of their sickle
shape -- and the parasite is eliminated along with them.
Glossary:
Hemoglobin- the part (protein) of blood that collects oxygen to give to the cells
in the body