Physical traits such as eye and hair colour are passed on by genes. Genes also produce proteins that are necessary for the body to function properly.
Every child inherits genes from its parents. A child receives 2 copies of each gene:
- One copy from the mother
- One copy from the father
Sometimes genes can be defective and pass on certain diseases. These are called “hereditary” or “genetic” diseases.
People who have only one copy of a defective gene that causes a disease are called “carriers”. They do not have the disease themselves but can pass their copy of the defective gene on to their children.
Two parents who are carriers of the same defective gene can each pass their copy of the defective gene on to their child. When this happens, the hereditary disease is called “recessive”.
Characteristics of a carrier of a defective gene associated with a disease
A person who is a carrier of a defective gene associated with a recessive hereditary disease:
- Has one copy of the defective gene responsible for the disease and another normal copy of the same gene
- Does not have the disease associated with the defective gene and is not at risk of developing it later
- Can have a child affected by the disease only if his or her partner also has the defective gene for the same disease. When both parents are carriers of the defective gene for the same disease, the possible outcomes for each pregnancy are as follows:
- 1 in 4 chance (25% probability) of having a child who is not affected by the disease and who is not a carrier of the defective gene
- 1 in 2 chance (50% probability) of having a child who is a carrier of the defective gene
- 1 in 4 chance (25% probability) of having a child who is affected by the disease
All human beings carry a certain number of defective genes. This means that hereditary diseases affect all populations worldwide.
When a small group of people moves away to form a new population, these people “transport” some of the genetic traits of the original population. Consequently, there is less genetic diversity in the new population than in the original population.
When the people that make up the new population carry genes for hereditary diseases, these diseases will tend to occur more frequently in the new population. This is called the “founder effect”.
Most of Québec’s population is descended from fewer than 10,000 ancestors who came over from France in the 17th century. Some of the founders of Québec’s population were carriers of defective genes that cause hereditary diseases.
The founder effect in the Saguenay–Lac-Saint-Jean, Charlevoix and Haute-Côte-Nord regions
The population of the Saguenay–Lac-Saint-Jean, Charlevoix and Haute-Côte-Nord regions has genetic traits that are the result of the founder effect. The founder effect was amplified in these 3 regions because they were remote from other regions in Québec and because the birth rate was very high there. That’s why some hereditary diseases are more common in these regions than in other parts of Québec.
People whose ancestors come from these 3 regions are more likely than the rest of Québec’s population to have inherited a defective gene associated with one of these 4 diseases:
- Congenital lactic acidosis
- recessive spastic ataxia of Charlevoix-Saguenay;
- Hereditary motor and sensory neuropathy with or without agenesis of the corpus callosum
- Hereditary type 1 tyrosinemia
These people have a 20% probability (1 in 5 chance) of being a carrier of a defective gene associated with one of these diseases. This probability exists for these people even if there have never been any children with these diseases among their relatives or in their immediate family.
Carrier testing is done to screen for the defective genes associated with these 4 recessive hereditary diseases. Screening is offered to people who have at least one biological grandparent who was born in the Saguenay–Lac-Saint-Jean, Charlevoix or Haute-Côte-Nord regions. To find out more about these tests, go to the page Maladies héréditaires récessives chez les personnes originaires du Saguenay–Lac-Saint-Jean, de Charlevoix et de la Haute-Côte-Nord (in French only).
Last update: July 9, 2020