Genomics 101: What is a carrier?

In this series, ‘Genomics 101’, we go back to basics and explore some of the most important topics in genomics. In this blog, we explain the term ‘carrier,’ and how it relates to genetic conditions. 

First things first, what is a carrier?  

In genetics, a ‘carrier’ is a person who carries a gene that can cause a health condition, but is not directly affected by the condition themselves. 

Even though they have inherited a condition-causing gene, they often don’t develop any symptoms of the condition. 

But, how does this happen? 

Well, humans have around 20,000 pairs of genes. One gene from each pair comes from our mother, and the other comes from our father.  

It is this unique combination of genes from both of our parents that makes up who we are, what we look like, and how we function.  

However, if a parent has a gene that causes a health condition, there is a chance they will pass this gene onto their child, just like any of their other genes.

If the child does inherit this condition-causing gene, they may sometimes develop the health condition, but not always. 

If a person has the gene in question but does not develop any symptoms, they would be known as a carrier. 

Why don’t carriers develop the condition? 

Interesting question! Whether a child develops a condition after inheriting the gene depends on the specific condition we're talking about, and whether the gene that causes it is ‘dominant,’ or ‘recessive’. 

What do dominant and recessive mean?  

As we said above, humans have thousands of pairs of genes, with one gene from each pair inherited from each of our parents. 

If a gene is dominant, only one copy of that gene needs to be present in the pair for it to be expressed. So, only one copy is needed for the trait associated with that gene to be present in the person. 

If a gene is recessive, it requires two copies in the pair to be expressed. 

You can learn more about this in our previous Genomics 101 blog: What are dominant and recessive genes? 

In short, all of this means that a child who inherits a gene for a dominant condition will most likely develop the condition, regardless of which gene is inherited from the other parent.  

If a condition is recessive, however, they will need to inherit two copies of the condition-causing gene, one from each parent. 

Can carriers pass on the gene to their children? 

Yes - carriers can still pass a condition-causing gene onto their children, just as they can pass on any other genes to their children. 

Whether their child develops the condition or not will depend on the gene they inherit from their other parent. 

For example, if both parents are carriers, there is a chance that they both pass on the recessive condition-causing gene to their child. In this case, the child will develop the full condition. 

If the child inherits the condition-causing gene from only one of their parents, they will be a carrier. 

Do you have an example? 

Sure! There are many conditions caused by recessive genes that lead to people being carriers. For example, sickle cell.

Sickle cell is a condition that affects the shape of person’s red blood cells, causing symptoms such as anaemia, painful episodes (sickle cell crises), and increased risk of infections. 

Sickle cell is particularly common in those with African and Caribbean family background, and has historically been under-researched, meaning there aren't many treatment options for people living with it. 

Being a carrier for sickle cell is often referred to as ‘sickle cell trait,’ and it creates a higher risk of having a child with sickle cell.  

Occasionally, people with sickle cell trait might need to take added precautions to stop themselves becoming unwell. 

You can learn more about sickle cell through our Diverse Data webpage.