Autosomal Recessive: Cystic Fibrosis, Sickle Cell Anemia, Tay-Sachs Disease
We inherit genes from our 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 do not affect an offspring's gender. "Recessive" means that two 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 do not know they carry a recessive gene for a disease until they have a child with the disease. Once parents have had a child with a recessive trait or disease, there is a one out of four, or a 25 percent, chance that with each subsequent pregnancy, another child will be born with the same trait or disorder. This means that there is a three out of four, or a 75 percent, chance that another child will not have the trait or disease.
The birth of a child with a recessive condition is often a total surprise to a family. In most cases, there is no previous family history of a recessive condition. Many autosomal recessive conditions occur this way. As mentioned above, a person who "carries" one copy of an autosomal recessive gene is usually not aware they carry the gene. They do not show any signs of the disease or condition. It is estimated that all people carry about five or more recessive genes that cause genetic diseases or conditions. Usually people do not know they carry a recessive gene, unless they have a child who is affected, or another family member.
What are autosomal recessive disorders?
Mutations (or changes in the DNA that codes for a gene) have occurred over time in different parts of the world. Anyone can carry virtually 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 found most commonly 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 descent.
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 one of the most common, inherited, single-gene disorders in Caucasians. About one in 3,000 Caucasian babies is born with CF. People with CF secrete abnormal body fluids. This includes unusual sweat and a thick mucus that prevents the body from properly cleansing the lungs. The mucus interrupts the function of vital organs and leads to chronic infections. Classic CF also involves the pancreas and causes decreased absorption of essential nutrients. Life expectancy for individuals with CF has improved. Ultimately, death most often occurs from respiratory failure. Other people with variants of CF may have only lung involvement, sinusitis, or infertility.
Sickle cell anemia
Sickle cell anemia is another of the most common, inherited, single-gene disorders in African-Americans. About one in 500 African-American babies is born with sickle cell anemia. About one in 12 African-American people carries the gene for this disease. Sickle cell disease involves the red blood cells, or hemoglobin, and their ability to carry oxygen. Normal hemoglobin cells are smooth, round, and flexible, like the letter "O." They can easily move through the vessels in our bodies. Sickle cell hemoglobin 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 cannot 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 is an organ that 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 properly. Without a functioning spleen, these individuals are more at risk for infections. Infants and young children are at risk for life-threatening infections. Treatment includes prompt emergency care for fevers and infections, appropriate vaccinations, 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. Dr. Tay and Dr. Sachs, who originally described this condition, noted that most Tay-Sachs babies were usually of eastern European Jewish origin. About one in 27 persons of Ashkenazi Jewish ancestry carries the Tay-Sachs gene.