When it comes to genetic risks related to a future child, the risks of trisomy 21 and other viable chromosomal abnormalities are usually widely discussed, and these prenatal screening tests are universally offered during pregnancy. In the vast majority of cases, these chromosomal abnormalities are accidental, and not inherited from the biological parents.
We hear less often about hereditary genetic risks, associated with the transmission of severe genetic conditions, which biological parents can silently carry. These conditions are generally referred to as recessive or X-linked, depending on how they are transmitted. Although these risks can have a significant impact on the health of a child, they are not systematically assessed during preconception or prenatal medical consultations.
Unlike the risk of trisomies, this hereditary genetic risk is not associated with maternal age and exists for all couples even in the absence of a family history. However, it can vary according to our ethnic or geographic origins and if we are related to our partner.
How being related impacts reproductive risk
We are all carriers of several silent pathogenic genetic variants without knowing it, and two biological parents who are carriers of the same genetic condition have a 25% (1/4) risk of transmitting these variants and having a child with the genetic condition in question1.
The greater the proportion of DNA shared between the two partners in a couple, the greater the risk that they will both carry variants in the same gene, and therefore have a child with a genetic condition.
It is common to choose a partner from an ethnic community similar to our own. When two partners are part of the same family (and are therefore directly related genetically) it is called consanguinity. This means that they share a larger proportion of genetic information (the closer the relationship, the greater the proportion).
Even if we differentiate between belonging to the same ethnic community and belonging to the same biological family, the same concepts apply here: the closer we are genetically to our partner, the higher the risk of carrying similar genetic variants and therefore having a child with a hereditary condition.
A couple who are biologically related will therefore have a higher risk than the general population of having a child with a hereditary genetic condition that could be silently carried by common relatives.
Founder effect
As mentioned above, not all ethnic populations are equal in terms of the risk of transmitting genetic conditions to children, and this is partly due to their demographic history. Certain populations are particularly affected by a specific risk of inheritance of genetic conditions. This is due to what is called a founder effect.
In some parts of the world, when a new population was established by a small number of individuals, they brought to the new region only a small part of the genetic diversity present in the original population. In addition to this loss of genetic diversity, there was usually geographical or social isolation, and a high birth rate, which gave the newly formed population a higher risk of carrying the few specific genetic changes that were present in the small initial group of “founders”.
A couple from the same community known to have a founder effect will therefore have a higher risk than the general population of having a child with one of the genetic conditions more common in that community.
This is particularly the case for a specific population in Quebec: people from the Charlevoix and Saguenay-Lac-Saint-Jean region. Individuals from this region are known to be at higher risk of carrying genetic variants for specific genetic conditions, due to their demographic history. For example, 1 in 23 people in this population silently carry a genetic variant for lactic acidosis, while this condition is rather rare in the rest of the world (less than 1 in 500 people are silent carriers). Similar examples exist for individuals of Ashkenazi Jewish origin, including Tay Sachs disease, with approximately 1 in 30 individuals of this background being silent carriers.
For two biological partners from the same region known to have a founder effect, genetic screening should be considered to ensure that they are not at increased risk of having a child with one of these inherited conditions.
A broader influence of our ethnicity on genetic risk
Beyond the founder effect, our ethnicity can largely influence which genetic conditions we are most at risk of silently carrying. An example is that of hemoglobinopathies, which are diseases of the hemoglobin, such as thalassemia and sickle cell anemia. Although present in all populations around the world, these conditions are found much more frequently in Asian, African, Caribbean, and Mediterranean populations. In contrast, for the Northern European and French-Canadian populations, cystic fibrosis is one of the most common conditions, with about 1 in 25 people being silent carriers without knowing it.
Choosing a reproductive partner of a different ethnic origin than our own can thus statistically reduce the risk of transmission of inherited genetic conditions, but this risk is never eliminated since many other diseases (e.g. spinal muscular atrophy, fragile X syndrome, etc.) are present in all populations.
All couples, regardless of ethnicity and family history, are at risk of having a child with an inherited genetic condition.2
What can I do to reduce my risk of passing on a hereditary genetic condition?
Rest assured, even if there is no such thing as zero risk, the majority of children are born healthy and do not have a genetic condition inherited from their biological parents, even if they come from the same population!
If you have concerns about your risk of passing on an inherited genetic condition, there are several points to consider:
Discuss with your relatives
The presence of a specific genetic diagnosis in the family can greatly impact your risk and can easily be assessed during a genetic counselling consultation3.
Obtain appropriate follow-up if needed
For couples who are related, beyond genetic risk assessment by a genetic counsellor and expanded carrier screening (see below), a specific prenatal ultrasound follow-up can sometimes be considered when a pregnancy is in progress.
Request genetic testing targeted to your origins
Targeted genetic testing is available in the public system for certain populations subject to the founder effect4. If you and/or your partner test positive and you are identified as a carrier of a genetic condition, then you will be referred for genetic counselling3.
Access expanded carrier screening
Unlike targeted public tests that are limited to a small number of conditions related to founder effects, the expanded carrier screening test is available privately and can screen for a larger number of silent genetic conditions (500+), including several dozen conditions associated with founder effects in certain populations. All couples or individuals, regardless of their ethnic origin and family history, can decide to access this test5. This testing should ideally be carried out before trying to conceive, to check that both partners are not carriers of genetic variants for the same condition, but the test can also be ordered at the beginning of an ongoing pregnancy.
Learn about your reproductive options
If you and your partner had been identified as carriers for the same condition, genetic counselling will be recommended to review your reproductive options. These usually include, but are not limited to, neonatal diagnosis (at birth), prenatal diagnosis (during a pregnancy), and pre-implantation diagnosis (on an embryo).
If you would like to schedule an appointment with one of our genetic counsellors, call us at 1-844-440-5454, send us a request online or book directly in our online portal.
If you are interested in accessing expanded carrier screening (available directly for general preconception where there is no family history or for egg or sperm donor choice), you can request it here or contact us for more information.
- For the sake of simplicity, this article focuses only on genetic conditions with an autosomal recessive inheritance.
- A 2021 study by an American laboratory (Invitae) analysed more than 22000 couples and found that nearly 4% of them were unknowingly carriers of genetic variants for the same diseases, i.e. at a 25% risk of having a child with a severe hereditary condition.
- Genetic counselling is available in the public system in most tertiary centres covered by provincial insurance. Private options exist to access these services more quickly in a preconception context.
- Access to public testing for people from the Saguenay-Lac-Saint-Jean, Charlevoix and Haute-Côte-Nord regions
- Access to public testing for individuals of Ashkenazi Jewish origin
- A doctor’s referral may be required. The presence of a family history of genetic condition should first be addressed during genetic counselling.