What is carrier screening?
Carrier screening involves a blood test to see if you and/or your partner carry a genetic change that could increase your chances of having a baby with a specific genetic condition. It is typically offered for a group of genetic conditions called “autosomal recessive” conditions.
What are autosomal recessive conditions?
Autosomal recessive conditions happen when an individual has a mutation in both copies of a particular gene. Since we inherit one copy of each gene from our mother and one copy from our father, the parents of an individual with a recessive condition are carriers of the condition. Carriers have a mutation in only one copy of the gene; therefore, they typically do not have the condition themselves, but they do have a chance of producing a child with the condition. Autosomal recessive conditions affect both males and females.
If only one parent is a carrier of a particular condition, the chance their children will have the condition is very low. However, if both parents are carriers of the same condition, the chance for each child to have the condition is 25%, whereas there is a 50% chance for each child to be a carrier, and a 25% chance for each child to neither have the condition, nor be a carrier.
What are some examples of autosomal recessive conditions that carrier screening is available for?
Cystic Fibrosis (CF)
CF is an inherited condition that causes a build-up of thick mucus in the lungs, leading to severe respiratory problems, and in the digestive tract, leading to difficulty in digesting and absorbing adequate nutrients from food.
Thalassemia is an inherited blood disorder in which the body makes an abnormal form of hemoglobin, the protein in red blood cells that carries oxygen. The disorder results in excessive destruction of red blood cells, which leads to anemia.
Sickle Cell Anemia
Sickle cell anemia is an inherited condition in which red blood cells form an abnormal crescent shape. The fragile, sickle-shaped cells deliver less oxygen to the body's tissues. They can also get stuck more easily in small blood vessels, and break into pieces that interrupt healthy blood flow.
Tay-Sachs disease is a rare, inherited disorder. It causes too much of a fatty substance to build up in tissues and nerve cells of the brain, causing mental and physical problems.
Who should consider carrier screening?
Some common reasons that an individual or couple might consider having carrier screening include:
- Increased chance of carrying an autosomal recessive condition based on ethnicity. If you are of Ashkenazi Jewish, French Canadian, Mediterranean, Middle Eastern, African, Southeast Asian, Western Pacific, Caribbean, or South American descent, you might be eligible for carrier screening for specific conditions.
- A family history of a known autosomal recessive condition.
- An ultrasound finding suggestive of an autosomal recessive condition.
I have no family history of genetic conditions – should I still consider carrier screening?
Yes. Many people who are carriers of an autosomal recessive condition will not have any affected individuals in their family. This is because a carrier must have a child with another carrier in order to have a chance of having an affected child. Remember, when two carriers have a child, there is a 25% chance of having an affected child, but there is also a 75% chance of having an unaffected child. Therefore, there may be generations of carriers but no affected individuals.
Is prenatal testing available for autosomal recessive conditions?
If both parents are found to be carriers of the same condition, there is a 25% chance that each of their children will have the condition. Assuming that the specific gene mutation(s) has been identified, prenatal diagnosis via chorionic villus sampling (CVS) or amniocentesis is typically available to see if the fetus/baby has the condition.
When should carrier screening be performed?
If carrier screening is appropriate for you, it should ideally be performed prior to conception so you and your health care provider can discuss the chances of having a child with an autosomal recessive condition. However, it can be performed during pregnancy, keeping in mind that pregnancy management options might become limited (for certain conditions) as your pregnancy progresses.
Interested in learning more information?
If you would like more information about carrier screening, speak with your healthcare provider to determine if a referral for genetic counselling would be appropriate for you.
Written by: Danna Hull, MSc, CCGC, CGC, Genetic Counsellor, Kingston General Hospital