Monogenic diseases result from alterations in a single gene. We all have two copies of each gene (allelic genes); one comes from our mother and the other from our father. Monogenic diseases may result by a mutation in a single gene. A monogenic disorder example is Achondroplasia, which results in dwarfism.
In other cases of monogenic diseases, mutations in both the allelic genes have to coexist for someone to be a patient. Such examples of monogenic diseases are Thalassemia a, Thalassemia b, or cystic fibrosis. If the mutation exists only in one of the two allelic genes, then this person is not considered patient, but a carrier of the disease. Carriers do not show symptoms of the disease. However, if both the partners are carriers of the same gene mutation, they have 25% chances of having a child suffering from the disease.
The hereditary pattern of each genetic disease is related to the possibility of inherit the disorder to the offspring, the extent of the symptoms or even the sex of the person that is most likely to suffer. An individual who is phenotypically normal and does not suffer from a hereditary disease, may be carrier.
As a carrier is considered a person who, while healthy, carries in his genetic material the mutant gene that causes the disease and can inherit it to his offspring. The probability of an individual- whose parent is a carrier- to be a carrier himself or to suffer from the disease or be absolutely healthy depends on the hereditary pattern of the particular genetic disease and of course on the genetic profile of the other parent, as well.
It is therefore important for a couple planning to have children to carry out all the necessary examinations to check if they are carriers of a hereditary disease. This examination is important to be performed at least for the diseases that occur with a relatively high incidence in the population (eg cystic fibrosis, thalassemia), regardless of the family medical history.
On the other hand, if it is known that there is a family history of a hereditary disease, or if one of the two candidate parents is a carrier, then it is very important for the couple to do all the necessary examinations. In addition, it is also considered necessary for couples who have already had children with a genetic disease, or have a history of recurrent miscarriages.
Genetic consultation by a highly qualified geneticist is required to guide the couple regarding all the tests they have to perform, for the interpretation of the results, and for the required prenatal control.
Nowadays, assisted reproduction also offers the possibility of Pre-implantation Genetic Diagnosis (PGD). Preimplantation Genetic Diagnosis is an invasive technique, especially recommended in cases in which one or both partners are carriers of a hereditary disease. This method makes it possible to examine embryos – resulting from an IVF process – for a particular hereditary disease, before the embryo transfer. The purpose of PGD is to examine the embryos and transfer only those that are healthy. In this way, couples that are carriers of a hereditary disease can now have children free from the disease.
Throughout this whole effort, couples always receive the necessary genetic counseling to be properly guided in each step .