What does it mean to be a carrier of a genetic disease?

Being a carrier of a genetic disease means that you have a variant related to a disease within your genetic material. This variant does not cause the disease and, generally, does not cause any symptoms, so why is it important to know if you are a carrier of a disease? In this article, we explain what it means to be a carrier and why this is important.

A brief introduction to genetics

As you may have read in other articles from our blog, DNA is a complex molecule that is found in practically every cell of our bodies and it contains the necessary instructions for creating, developing and reproducing life. Within DNA we can differentiate specific fragments, known as genes, which contain the necessary information to give rise to the proteins that carry out many functions in the body.  

The complete set of our DNA is called a genome. DNA is found packed into cells and its maximum degree of compaction is reached during the process of cellular division where the chromosomes appear. In general, human beings have 23 pairs of chromosomes, that is, 46 chromosomes in total, 23 of which are inherited from our father and 23 from our mother. This means that, as we have two copies of each chromosome, we have two copies of all our genetic information and, therefore, two copies of each gene.

As you may have seen for yourself, offspring resemble their parents and, as we have said, this is due to the DNA inherited from parents to their offspring, a process known as genetic inheritance. Imagine the genome has an instruction manual that we inherit from our parents and which determines everything from the colour of our eyes, our height, and even features that, as well as being genetic, also depend on other factors such as lifestyle and environment. But we don’t just inherit our features, we also inherit diseases and, on occasion, it may be the case that many members of a family present with the same disease.

Monogenic and inherited diseases

As we mentioned above, DNA is our manual and therefore an error, commonly known as a “mutation”, may alter some of the instructions in this manual and ultimately cause disease.   

On a genetic inheritance level, the diseases with the greatest impact are the monogenic diseases. With these pathologies, an alteration to a specific gene may result in disease. There is a key factor in this type of diseases: the mode of inheritance. In monogenic diseases, we can distinguish two principal modes of inheritance:  

• Dominant inheritance: in these cases, only one altered copy of the gene is enough to cause disease.
• Recessive inheritance: both copies of the gene have to be altered for disease to appear. If we only possess one copy of an altered gene, we won’t develop the disease, but we will be carriers.

The importance of knowing your status as a carrier

As we have mentioned, by being a carrier of a recessive disease, generally, we won’t present with any symptoms of that disease. Even so, it is important to be aware if we are carriers when starting a family. Why? The reason is that if our reproductive partner is also a carrier of the same disease, there is a high probability that offspring will inherit both copies of the altered gene from the parents and, in this case, they will present with the disease. Here lies the importance of being aware of our carrier status.  

There are some terms that are worth clarifying in order to talk about “carriers”. When we want to refer to whether a person is a carrier, it is generally referred to as “carrier status”. Another term that we might hear is “carrier frequency”, these terms refer to how common it is to be a carrier of a specific disease. Carrier frequency differs depending on the disease. If a disease has a carrier frequency of 1 in 25 it means that, even if you are not aware of it, 1 out of every 25 people will be a carrier of that disease.

The most common recessive diseases

Although it may depend on the geographical region and the population, there are recessive diseases with greater carrier frequency. This increases the probability of both partners being carriers of the same disease and, therefore, having a child that develops the disease.

Some diseases that have a high carrier frequency are:  

• Cystic fibrosis: is a disease that affects almost 1 in every 4,500 people and its carrier frequency is estimated to be 1 in every 25 people. It is a multisystem disease that affects various parts of the body, mainly the respiratory and digestive systems.
• Phenylketonuria: is a disease that affects 1 in every 10,000 newborns in Europe. Sufferers are unable to break down a molecule called phenylalanine, and this accumulates in the body causing different symptoms, such as neurological problems or delayed development, among others. Carrier frequency for this disease in certain countries such as Ireland and Turkey is approximately 1 in every 30 people.  
• Haemochromatosis: it is estimated that, in Caucasians, it affects 2-5 people in every 1,000. This is an iron metabolism disease that causes excess deposits of iron in the body. It is estimated that carrier frequency is 1 in every 10-12 people.
• Spinal muscular atrophy: 1 in every 40-50 people is a carrier of this disease. It belongs to a heterogeneous group of pathologies known as muscular dystrophies in which there is progressive damage of the lower motor neurons. It is estimated to affect 1 in every 6,000 newborns worldwide.

Today, we have genetic tests that enable us to find out if we are carriers of recessive diseases. When both partners are carriers of the same disease, the risk of descendants developing that disease is 25% for each pregnancy. In other words, being a carrier does not mean that the person presents with a disease, but it does mean that there is a risk of passing on that disease to your offspring.  

At Veritas, we support proactive healthcare at all stages of life. We have a wide range of genetic tests and a team of clinical geneticists to offer advice on the most suitable test for each case. If you would like further information, please don’t hesitate to contact us.