Some people have a higher risk of having a child with a genetic condition because they carry certain changes in their genes (called gene mutations or gene variants). People who have a gene mutation but don’t have the genetic condition caused by the altered gene, are called “carriers”.
A doctor or genetic counsellor may talk to you about carrier testing if you have a family history of a certain genetic condition or an ancestry that has a higher rate of a certain genetic condition.
Most often, genetic testing for carrier status (also called carrier testing) can find out if a healthy person is carrying a specific gene mutation for a genetic condition. Information from this type of testing can help you understand the chance of passing on a gene mutation to children. Some questions that carrier testing can help answer include:
- Am I at risk for sickle cell disease because my ancestors were from Africa or cystic fibrosis because I am of European descent?
- My doctor told me that I may have a gene for alpha thalassemia, even though I am healthy. What does this mean?
- Do I have a chance of having a child with the same rare genetic condition as my family member?
What does it mean to be a carrier for a genetic condition?
Genes are made of DNA (deoxyribonucleic acid) and act as instructions for the way our body works and develops. These instructions also make us unique and give us our features such as different eye colors and shapes. We have about 20,000 genes. We inherit 1 copy of each gene from our mother and 1 copy from our father.
Gene mutations are small changes in the DNA that makes up a gene. They change how a gene works and how our bodies work and develop. There are many genetic conditions caused by gene mutations but they aren’t all inherited the same way.
Most people have 2 copies of each gene. For some genetic conditions, you only need
1 copy of a gene with a mutation to have the genetic condition related to that gene. This is called autosomal dominant inheritance. In most cases, if you carry a gene mutation that is inherited in an autosomal dominant way, you will develop the genetic condition at some point.
For other conditions, you need to have mutations in both copies of the same gene to develop the genetic condition. This is called autosomal recessive inheritance. In this case, each parent has 1 copy of the gene that has a mutation, and a second copy of that gene with no mutation. The parents are “carriers” of the genetic condition. In most cases, they would have no signs or symptoms of the genetic disease. We are all carriers for some autosomal recessive genetic conditions.
When both parents are carriers for the same autosomal recessive condition, for each pregnancy there is a:
- 1 in 4 (25%) chance that their child will have the genetic condition.
- 2 in 4 (50%) chance that their child won’t have the genetic condition but will also be a carrier of the genetic condition. Most carriers are healthy.
- 1 in 4 (25%) chance that the child will not inherit a mutation in the gene from either parent, and won’t have either the genetic condition or be a carrier for that condition.
If only one parent carries a gene mutation for a genetic condition, the child has a 1 in 2 (50%) chance of being a carrier but almost no chance that they will have the disease.
Talk to your doctor if you have a family history of a genetic condition or would like to know if you have a higher risk of being a carrier for an autosomal recessive condition because of your ancestry. It is most helpful to talk about your family history well before you plan your pregnancy.