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

Being a carrier of a genetic disease implies that we have a variant in our genetic material related to a disease. This variant does not cause the disease and generally does not cause symptoms either, but then why is it important to know if we are carriers of a disease? In this article we explain what it means to be a carrier of a disease and what its importance is.

A brief introduction to genetics

As you have already been able to read in other articles on our blog, DNA is a complex molecule that is found in practically all the cells of our body and contains the necessary instructions for the creation, development and reproduction of life. Within DNA we can differentiate specific fragments called genes, which contain the necessary information to give rise to proteins that carry out multiple functions in the organism.

The complete set of our DNA is called a genome. DNA is compacted inside cells and its maximum degree of compaction is reached during the cell division process where chromosomes appear. Human beings generally have 23 pairs of chromosomes, that is, 46 chromosomes in total, 23 inherited from our father and 23 from our mother. This means that, by having two copies of each chromosome, we will have two copies of all our genetic information and, therefore, two copies of each gene.

As you may have observed, children resemble their parents and this is because, as we have said, DNA is inherited from parents to children, which is known as genetic inheritance . We can imagine the genome as the instruction book that we inherit from our parents and it determines everything from our eye color, hair color, height, to traits that, in addition to having a genetic part, also depend on other factors such as lifestyle or environment. . But not only our traits are inherited, diseases can also be inherited and sometimes it can happen that many members of a family present the same disease.

Monogenic and hereditary diseases

As we have already mentioned, DNA is our instruction book and, therefore, an error, commonly known as a “mutation”, can alter any of the instructions in this book and ultimately cause disease. 

At the level of genetic inheritance, the diseases with the greatest impact are monogenic diseases, in these pathologies an alteration in a specific gene can give rise to the disease. In this type of disease there is a key factor: the mode of inheritance. In monogenic diseases we can distinguish two main modes of inheritance:

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

The importance of knowing our carrier status

As we have said, being a carrier of a recessive disease, generally we will not present any symptoms of the disease. Even so, it is important to know if we are carriers in order to start 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 the children will inherit the two altered copies of the gene from their parents and in that case they will present the disease. Here lies the importance of knowing our carrier status.

There are some terms that are worth clarifying when talking about “carriers”. When we want to refer to whether a person is a carrier of a disease, we generally speak of “carrier status”. Another term you may hear is “carrier rate” or “carrier frequency,” these terms refer to how common it is to be a carrier for a particular disease. The carrier rate is different depending on the disease. If a disease has a carrier rate of 1 in 25 it means that even if you don’t know it, 1 in 25 people will be a carrier of the disease.

Most common recessive diseases

Although it depends on the geographical area and the population, there are recessive diseases with a higher carrier rate. This increases the chances that both members of the couple will be carriers of the same disease and, therefore, of having a child who will develop the disease.

Some of the diseases that have a high carrier rate are:

  • Cystic fibrosis: this is a disease that affects about 1 in 4,500 people and its carrier rate is estimated at 1 in 25 people. It is a multisystemic disease that affects various parts of the body, mainly the respiratory and digestive systems.
  • Phenylketonuria: is a disease that affects 1 in 10,000 newborns in Europe. People who suffer from it are not able to break down a molecule called phenylalanine, which accumulates in the body, causing different symptoms such as neurological problems or delayed development, among others. The frequency of carriers of this disease in certain countries such as Ireland or Turkey is approximately 1 in 30 people.
  • Hemochromatosis: it is estimated that in Caucasians it affects 2-5 people per 1,000. It is a disease of iron metabolism that causes an excessive deposit of iron in the body. It is estimated that the carrier rate is 1 in 10-12 people.
  • Spinal muscular atrophy: 1 in 40-50 people is a carrier of this disease. It belongs to a heterogeneous group of pathologies called muscular dystrophies in which there is a progressive destruction of lower motor neurons and it is estimated that it affects 1 in 6,000 newborns worldwide.

Today we have genetic studies that allow us to know if we are carriers of recessive diseases. When both members of the couple are carriers of the same disease, the risk of the offspring developing the disease is 25% in each pregnancy. In other words, being a carrier does not imply that the person has a disease, but it does imply a risk of transmitting said disease to offspring.

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