What is Genetic Carrier Screening & How is it Done?

In recent times, more and more doctors, GPs, and OBGYNs are recommending genetic carrier screening for all couples planning pregnancy. But what exactly is a carrier screening test? How is it done?

Carrier screening is an important prenatal genetic test. Taking a test with your partner can help you understand your chance of having a child affected with a serious genetic condition.

Given the recent changes to Medicare, many Australians are understandably looking into carrier screening and wondering how it works. Fortunately, while the science behind carrier screening may be complicated, the process for a patient is quite simple.

Here's how it works.

How Does Carrier Screening Work?

Your genes are like the instructions for our body. Sometimes, they have small changes or "variants" that can affect how our body works. Most people have some changes in their genes, but not all of these changes cause health problems.

Genetic carrier screening is a type of prenatal testing that can check what variants you have, and therefore what you can pass onto any children you have.

To understand reproductive carrier screening, we first need to know a little bit about gene variants and inheritance.

Gene variants

Everyone has 2 copies of each gene; one you've inherited from your mother and one from your father.

Often, you'll see geneticists label these gene variants with either a capital letter ("A") for dominant genes, or a lowercase letter ("a") for recessive genes. This means that the possible combinations are:

• Two dominant genes (AA),
• One dominant and once recessive (Aa), or
• Two recessive genes (aa).

The variant you have will depend on what you inherit from your parents. If both your parents are AA, then you can only inherit an A from both parents (meaning you'll be AA as well).

However, if your parents have a mix of AA, Aa, and aa, then you could inherit any of the above combinations.

Where this becomes important is when we look at dominant and recessive conditions.

Dominant vs. Recessive conditions

For autosomal conditions, there are 2 types of inherited conditions: dominant and recessive.

For an autosomal dominant condition, having just a single altered copy of the gene is enough to be affected by the condition. For an autosomal recessive condition, you will need to have altered variants in both copies of the gene.

Since you only need one altered gene copy to be affected by a dominant condition, carrier screening isn't necessary for dominant conditions (you'll already know your reproductive risk from having the condition).

When it comes to carrier screening, it will instead look at recessive conditions. For recessive conditions, since you can have one changed gene (Aa) without having the condition, it's possible to be a "carrier" for the condition.

What does it mean to be a "carrier"?

Carriers usually don’t have the health issue themselves, but they can pass on the changed gene when they have children.

Thinking back to our gene variations:

• If you have two normal genes (AA), you won't have the condition and aren't a carrier.
• If you have one of each (Aa), you won't be affected by the condition, however you are a carrier and could pass it onto your children.
• If you have two changed genes (aa), you will have the condition.

Being a carrier is usually "silent", meaning you most likely will have no idea that you are a carrier. However, it becomes extremely important to know if you are a carrier when you plan on starting a family.

Carrier Screening & Family Planning

When two people are planning a family, knowing what gene copies they have helps them to be more informed on the chance of their child having a condition.

There is a medical test that can help called a "carrier screening test".

Like the name implies, a genetic carrier screening test will look at your DNA and identify if you are a carrier for a panel of genetic conditions. The test examines the genes in your DNA and identifies any changed genes that might result in you passing a condition onto your child.

The test works by comparing your DNA to a reference set of data to identify where the changes are. You can learn more about the science and technology behind carrier screening testing here.

Carrier screening works best when your reproductive partner is also tested. If you both know your carrier statuses, you can understand your shared risk of having a child with a condition. Often, tests such as the Lumi Extended or Comprehensive Carrier Screening Test provide a "couples' report" that highlight this combined chance.

In order to pass on the condition, you and your partner both need to be carriers for the same condition.

To explain, let's take another look at our gene variants.

• AA + AA

  If both parents are AA, then their children will also have AA gene copies. These children won't have the condition.

• AA + Aa

  If one parent has AA and the other has Aa, their children will have either AA or Aa gene copies.

  Since carrier screening looks at recessive conditions, these children won't have the condition (but may also be carriers if they are Aa).

• Aa + Aa

  If both parents are carriers, meaning they both have Aa gene copies, their children have:

  • 50% chance of being a carrier with Aa gene copies.
  • 25% chance of being unaffected with AA gene copies.
  • 25% chance of aa gene copies, meaning they've inherited the condition.

For patients who find out they are carriers, it therefore does not mean that they can't or shouldn't have children. Even if both you and your partner are carriers for the same condition (and therefore have an increased chance of having an affected child), there are options available to help ensure you have a healthy pregnancy.

Often genetic counsellors can help explain what your results mean, what impact they may have, and what options are available to you.

What does carrier screening test for?

 

How is carrier screening done?