It is possible to be a carrier of a genetic disease and not know it. This is because carriers of recessive genetic disorders generally do not have any symptoms of the disease. However, they can transmit the gene mutation to their children. That is why your healthcare provider may suggest you and your partner do a genetic screening. These screening tests can confirm a family’s likelihood of having a child with a serious inherited disease.
How a baby may inherit a recessive genetic condition
When a disorder is recessive, the baby must inherit the defective gene from both parents to be affected (see image below using cystic fibrosis as an exmple). If only one parent has the defective gene, the baby has 50% chance of inheriting it (unless the parent has the disease, and then the baby will surely inherit it from that parent). For a recessive disease, a baby with maternal and paternal defective copies of the gene will have the disease. If the baby has only one copy of the gene, he or she will be a carrier and will not show any effects of the disease. If both parents have a copy of the disease gene (like in the image below), all their children have 25% chance of being unaffected and not a carrier, 50% chance of being unaffected and a carrier, and 25% chance of being affected. That is, 75% chance of not having the disease and 25% of having it. Again, this is for each pregnancy they have.
Tests for recessive genetic conditions
Parents can be tested before deciding to have a baby, as well as during a pregnancy. Usually, the mother is tested first and, if she tests positive as a carrier of the defective gene, then the other parent is tested. This screening is offered to individuals who have a family history of genetic disorders or those who belong to an ethnic group with a higher than average risk. This is the reason why your healthcare provider will probably ask you about your family background.
For example, when a woman with an Eastern European Jewish background (known as ashkenazi) wants to have children with her French Canadian partner, they have a higher than average risk of having a child with a Tay-Sachs disease. This is because both groups (ashkenazi and French Canadian) have an increased risk of carrying the Tay Sachs gene. As explained above, if only one parent is a carrier, their child will not have the disease.
Knowing your risks can help you make an informed decision about family planning.
Examples of recessive genetic conditions
Here are some recessive disorders and the groups at risk.
- Cystic fibrosis. More than 10 million Americans are carriers of one mutation of the cystic fibrosis gene, a life-threatening disease that causes lung damage and digestive disorders. Although European Caucasians and Eastern European Jews have an increased risk of having a child with cystic fibrosis, guidelines from the American Congress of Obstetricians and Gynecologists favor testing for any woman who is thinking about getting pregnant. Since testing became more widespread, the incidence of babies born with cystic fibrosis has dropped significantly.
- Tay-Sachs: In the U.S. today, approximately one in every 27 Jews is a carrier for Tay-Sachs, an inherited disorder that progressively destroys nerve cells in the brain and spinal cord. Babies born with Tay-Sachs may seem normal at birth but at six months begin to show symptoms. This genetic mutation is more common in people of Ashkenazi (eastern and central European) Jewish heritage but is also are found in certain French-Canadian, Amish and Cajun communities. Those of Eastern European Jewish heritage also face a higher risk of two other degenerative inherited nerve diseases, Familial Dysautonomia and Canavan disease.
- Sickle Cell disease: As many as 100,000 Americans have sickle cell anemia the most common form of an inherited blood disorder that leads to a weakened immune system. People with sickle cell disease have abnormal hemoglobin, the protein in red blood cells that carries oxygen throughout the body. Treatment can help sickle cell disease but it can’t be cured and may lead to organ damage. The inherited disorder is most common in people of African, Mediterranean and Southeast Asian background.
- Thalassemia is also an inherited blood disorder in which the body makes an inadequate amount of hemoglobin. The disorder, which can be treated but not cured, leads to anemia, greater risk of infections and possible bone deformities. Thalassemia is most often found in African-Americans and people of Mediterranean or Southeast Asian ancestry.
If you plan to start a family, it’s a good idea to talk to your healthcare provider about genetic screening for recessive disorders.
Image credit: https://opentextbc.ca/anatomyandphysiology/.