Autosomal Recessive: Cystic Fibrosis, Sickle Cell Anemia, Tay-Sachs Disease
You inherit genes from your biological parents in specific ways. One of the ways is
called autosomal recessive inheritance.
What is autosomal recessive inheritance?
Autosomal recessive inheritance means that the gene in question is located on one
of the autosomes. These are numbered pairs of chromosomes, 1 through 22. Autosomes
don't affect an offspring's gender. "Recessive" means that 2 nonworking copies of
the gene are necessary to have the trait or disorder. One is inherited from the mother,
and one from the father. If you have only one recessive gene, you are a "carrier"
for the trait or disease, but you do not have any health problems from "carrying" one
copy of the gene. Most people don't know they carry a recessive gene for a disease
until they have a child with the disease, or they have another family member with
the disease. It's estimated that all people carry about 5 or more recessive genes
that cause genetic diseases or conditions. Once parents have had a child with a recessive
trait or disease, there is a 1 out of 4 chance that, with each subsequent pregnancy,
another child will be born with the same trait or disorder. This means that there
is a 3 out of 4 chance that another child will not have the trait or disease.
What are autosomal recessive disorders?
Mutations (or changes in the DNA) have occurred over time in different parts of the
world. Anyone can carry any type of recessive gene. However, certain ethnic groups
are more likely to carry certain recessive genes because of where the mutation originated.
For example, the gene that causes Tay-Sachs disease is commonly found in people of
Ashkenazi Jewish descent. This population is from Eastern Europe, where experts believe
the mutation originated. The disease also affects those who are not of Ashkenazi Jewish
What are some of the different types of autosomal recessive disorders?
Examples of autosomal recessive disorders include cystic fibrosis, sickle cell anemia,
and Tay-Sachs disease.
Cystic fibrosis (CF) is a common, inherited, single-gene disorder that affects a protein
in the body. CF may occur in people of all ethnicities. But it is mainly found in
whites. People with CF make abnormally thick and sticky mucus that can lead to blockages
and damage of body organs. The mucus interrupts the function of vital organs, especially
the lungs, and leads to chronic infections. CF also involves the pancreas and causes
decreased absorption of essential nutrients and can cause reproductive system damage.
Ultimately, death most often occurs from respiratory failure. Other people with variants
of CF may have only lung involvement, sinusitis, or infertility. CF has no cure, but
better treatments are helping people live longer and healthier lives.
Sickle cell anemia
Sickle cell anemia is another common, inherited, single-gene disorder found mostly
in African Americans. About 1 in 365 African-American babies is born with sickle cell
anemia. About 1 in 13 African Americans carries the gene for this disease. About 1
in 16,300 Hispanic American babies are born with sickle cell anemia. Sickle cell disease
involves the hemoglobin in the red blood cells and their ability to carry oxygen.
Normal red blood cells are smooth, round, and flexible, like the letter O. They can
easily move through the vessels in our bodies. Sickle cells are stiff and sticky.
When they lose their oxygen, they form into the shape of a sickle, or the letter C.
These sickle cells tend to cluster together and can't easily move through the blood
vessels. The cluster causes a blockage and stops the movement of healthy, normal,
oxygen-carrying blood. This blockage is what causes the painful and damaging complications
of sickle cell disease.
Sickle cells live only for about 15 days. Normal hemoglobin cells can live up to 120
days. Sickle cells risk being destroyed by the spleen because of their shape and stiffness.
The spleen helps filter the blood of infections. Sickle cells get "stuck" in this
filter and die. Due to the decreased number of hemoglobin cells circulating in the
body, a person with sickle cell is chronically anemic. The spleen also suffers damage
from the sickle cells, which block the healthy oxygen-carrying cells. After repeated
blockages, the spleen is very small and does not work correctly. Without a functioning
spleen, these people are more at risk for infections. Infants and young children are
at risk for life-threatening infections. Treatment includes speedy emergency care
for fevers and infections, appropriate vaccines, penicillin, and management of anemia.
Tay-Sachs disease is a fatal disorder in children (usually by age 5) that causes a
progressive degeneration of the central nervous system. It is caused by the absence
of an enzyme called hexosaminidase A (or hex A). Without hex A, a fatty substance
builds up on the nerve cells in the body, particularly the brain. The process begins
early in pregnancy when the baby is developing. It is not apparent until several months
after the birth. To date, there is no cure for Tay-Sachs. About 1 in 27 persons of
European Ashkenazi Jewish ancestry carries the Tay-Sachs gene.