What is Genetic Carrier Screening and How is it Done?

Genetic carrier screening is an important genetic test that helps with family planning. It helps couples understand their risk of having a child with an inherited genetic condition.

Prenatal testing is increasing in popularity among couples planning pregnancy due to becoming more affordable, simple, and easy to access. But how does a carrier screening test work? How is it done?

Here's how.

How does carrier screening work?

Reproductive carrier screening is a test that looks at certain genes that might cause health issues if a specific variant is passed on to your children.

Genes are like instructions in our body, and sometimes they have small changes that can affect how our body works. Most people have some changes in their genes, but not all of these changes cause health problems.

Part of why carrier screening is so important is because of certain conditions known as "recessive conditions". You can be a carrier for these kinds of conditions, even with no known family history.

But wait, what is a recessive condition? And what does it mean to be a carrier?

Let's take a deeper dive into how genetics works.

Recessive vs Dominant conditions

For inherited conditions, the 2 most common types are autosomal recessive and autosomal dominant.

When you're born, you inherit 2 copies of each gene – one from your mum and one from your dad. For dominant conditions, you only need 1 of these inherited genes to be the "changed" version, and you will have the condition. Carrier screening does not test for these conditions as you either have the condition or you don't.

However for recessive conditions, you need both (2) copies of the changed version to have the condition.

This means if you inherit only 1 changed gene for a recessive condition, you won't have the condition yourself but you will be what's known as a "carrier" of this condition.

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

Since you have a copy, it's possible for you to pass that gene onto your own children. If your reproductive partner has the same changed gene and also passes on the changed gene, then your child will have the condition – even if neither you nor your partner had it!

This is why it's important to know if you have this "silent" carrier status. That's where carrier screening testing comes in.

But how does carrier screening work out if you have an increased chance of having an affected child?

Inheriting genes & how carrier screening determines risk

There's a common method geneticists use when mapping out inheritance for recessive conditions.

Typically, they use capital letters like "A" for the dominant gene and lowercase letters like "a" for the recessive gene.

If you have two normal genes (AA), you won't have the condition. If you have one of each (Aa), you're a carrier. If you have two changed genes (aa), you will have the condition.

When two people are planning a family, knowing what gene copies they have helps them understand their combined chance of their child having a condition.

If both parents are AA:

• They will only pass on dominant (A) gene copies.
• This means their children will also be AA and will not have (or carry) the condition.

If one parent has AA and the other has Aa:

• Their children will have either AA or Aa gene copies.
• AA children will not have (or carry) the condition.
• Aa children will not have the condition but will be carriers and could pass it on to their future children.

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

• 50% (1 in 2) chance of being a carrier with Aa gene copies.
• 25% (1 in 4) chance of being unaffected with AA gene copies
• 25% (1 in 4) chance of aa gene copies, meaning the child will inherit the condition.

How is carrier screening done?

For a patient, there are several different ways of taking a carrier screening test. It usually depends on the provider you choose. For example, Lumi Health uses a simple, non-invasive cheek swab you can do at home. Other providers commonly use saliva or blood tests.

Once the sample has been obtained, it is transported to a laboratory so that testing can begin. There are several steps involved in testing, including DNA extraction, analysis, bioinformatics, and reporting.

Laboratories have high-tech scientific equipment that can extract certain parts of your DNA and turn it into ordered segments known as exomes. This process is called "sequencing".

They can then compare the order of your DNA's segments against reference orders to identify any specific changes in sequences that are known to be associated with genetic conditions.

These results are then interpreted by a geneticist who, with the help of genetic counsellors, can explain what the results mean and what impact this will have.

What does genetic carrier screening test for?

Carrier screening tests for recessive conditions to determine your chance of having a child affected by an inherited genetic disorder.

The specific genes and conditions varies depending on provider and test, but common conditions to test for include:

• Cystic Fibrosis (CF),
• Spinal Muscular Atrophy (SMA), and
• Fragile X Syndrome (FXS).

These conditions are all covered in the Lumi Standard Carrier Screening Test, which is bulk-billed through Medicare. The Comprehensive Carrier Screening Test covers an additional 1,200+ genes.

Where can I take a test?

These days, it's quite easy for any couple planning pregnancy to take a carrier test. Some providers such as Lumi Health allow you to order online and take a test from home, with results delivered in an online portal. Lumi offers Medicare bulk billing and doesn't require a referral from a doctor.

Otherwise, your doctor or GP will most likely be able to recommend a pathology provider and write you a referral for testing. You can then take this referral to a collection centre to take a test.

It's important to weigh up the differences between providers and tests to ensure you choose the test that's right for you. You can learn more about how to compare carrier screening tests here.