Cystic fibrosis : My family does not have a history of this disease, so I am not at risk of contracting it
It's a myth!
Indeed, it is not that simple! There are several modes of transmission for genetic diseases, some of which have an “autosomal dominant” pattern of inheritance. These diseases affect people carrying the mutant gene in each generation, and in this particular case, it is true that it is easy to assess the risk of contracting it based on family history.
However, a large percentage of genetic diseases are transmitted through an “autosomal recessive” pattern of inheritances: in order to be affected, an individual must receive a mutant gene from both parents; who are carriers of the disease but generally do not have any symptoms. For this reason, the disease can remain hidden for generations within a family, before a person is diagnosed with the disease.
Some examples of autosomal recessive diseases include:
It is estimated that one in 25 people in the Caucasian population carries this disease. The risk for a couple (without a family history) to have an affected child is therefore one in 3600 in Canada. In Quebec, where the population is mostly Caucasian, it is one in 2500 children. The difference between the two values reflects the greater homogeneity in the Quebec population.
One in nine people (in Caucasians) carries this disease, characterized by excess absorption and storage of iron in the body. The risk for a couple without a family history of having an affected child is one in 327.
Sickle Cell Anemia
Among African Americans, it is estimated that one in 10 people carry this disease. The risk for a couple (without a family history) to have an affected child is therefore one in 400.