It’s enormously common in medicine for a disease or quirk to “run in families” – for example, one’s risk of a certain type of cancer is elevated when a parent was diagnosed with it. But though there’s clearly a genetic component to many diseases, genetics alone don’t determine whether a person will or will not develop them.
Autosomal recessive disorders are different. They are a class of genetic disorders in which genetics alone determine whether an individual will develop a disease. In order to develop an autosomal recessive disorder, the child must inherit two copies of a mutated gene: one from each parent. That means that each parent must be carrying at least one copy of this mutated gene for their child to be born with the disorder.
In the case of any autosomal recessive disorder, a person is either:
- Not a carrier (carrying two normal genes; i.e. zero copies of the mutated gene),
- A carrier (carrying one normal gene, plus one copy of the mutated gene),
- Positive for the disorder (both their copies of the gene are mutated).
People who are carriers will not suffer from the condition themselves. The normal gene is dominant over the mutated one, which means that their health will usually not be affected. But if a carrier’s partner is also a carrier, their child has a 1 in 4 chance of developing the disorder.
The Most Common Autosomal Recessive Disorders
Many autosomal recessive disorders are much more common within certain ethnic groups.
- Cystic Fibrosis (CF) is most commonly found among Caucasians. Those who suffer from cystic fibrosis produce an abnormally thick and sticky mucus that can damage internal organs. The mucus also interrupts the function of vital organs, primarily the lungs, which leads to chronic infections. CF can also damage the pancreas and affect absorption of nutrients, as well as cause some damage to the reproductive system. Though sufferers of this condition generally live into adulthood and can lead fulfilling lives, the disease contributes to a reduced lifespan and respiratory failure is a common cause of death.
- Sickle Cell Anemia (also called sickle cell disease) is most commonly found among individuals of African descent. In the United States, around 1 in 12 African-Americans are carriers. The disease causes red blood cells to take on a malformed, “sickle” shape, which interferes with their ability to successfully transmit oxygen around the body. This causes chronic fatigue. Patients with sickle cell anemia are also more prone to infection, and may experience painful episodes called “sickle cell crises.”
- Tay-Sachs Disease is a fatal disorder that causes a progressive degeneration of the central nervous system. This disorder is caused by the absence of an enzyme known as hexosaminidase A (hex A), which causes a fatty substance to build up on the nerve cells in the body, especially the brain. Infants with Tay-Sachs may experience symptoms like decreasing muscle strength, muscle paralysis, seizures, deafness, and blindness.The symptoms begin in early pregnancy while the baby is still developing, but may not become apparent until several months after birth. Most children unfortunately do not survive past the age of five. Currently, there is no cure for this condition.
Understanding the Risk
Genetic carrier testing is an option for adults who may have a family history of an autosomal recessive disorder, and want a better understanding of what their child’s risk may be. This type of screening requires only a blood test, and will reveal whether either partner is a carrier for any number of autosomal recessive disorders. This type of screening can be performed either before or during pregnancy.
It’s also possible for genetic tests to be performed during pregnancy, which will show definitively whether the child has any genetic disorder. These tests are known as amniocentesis (a sample of the amniotic fluid is taken) and chorionic villus sampling (a sample of the placenta is taken). It should also be noted that both options are invasive, and do carry a small risk of inducing a miscarriage.
Speaking to your general practitioner, OBGYN, or genetic counselor about you and your partner’s’ risk of having a child with a genetic condition can help you learn more about what to expect from the testing process. Options are available, both invasive and minimally-invasive, no matter what stage of pregnancy.
Resources:
https://medlineplus.gov/ency/article/002052.htm
https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentID=P02142&ContentTypeID=90
http://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/home/ovc-20303267
http://www.healthline.com/health/tay-sachs-disease#symptoms2