Canada: inbreeding at the origin of the high rate of rare diseases in Quebec, reveals a study

While a handful of settlers in the seventeenth and eighteenth centuries gave birth to many descendants in Quebec, a specialist in population biology has just made a discovery on the origin of people who live in regions far from major centres. In a study published in Science, a mathematical model seems to reveal that the genetics of 2/3 of the current population came from only 2,600 ex-settlers, which would have favored the strangely high rate of a rare blood disease in this region.

According to the mathematical model established in the study, a small part of the 2600 settlers arriving in Quebec settled near Lac Saint-Jean, restricting any possible genetic mixing. © Cal Woodward / AP

Text by: Pascale Guéricolas


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From our correspondent in Quebec City,

Simon Gravel, a professor at McGill University in Montreal, had the revolutionary idea of cross-referencing the archives of marriage records in the Lac Saint-Jean region with databases on the DNA of Quebecers in this corner of the Canadian region.

His research, published by the prestigious magazine Science, shows that 2/3 of the current genetic heritage comes from only 2,600 people who have populated this territory. In other words, there was very little mixing of populations among the people who founded the villages around the huge Lac Saint-Jean.

We already know that a large proportion of Quebecers are descended from 8,500 French people out of 10,000 who arrived in the seventeenth and eighteenth centuries. Founders often came from the west of the France. At first, these first settlers mixed when they lived in Quebec City. Then, several families settled in Lac Saint-Jean further east, and there this genetic mixing ended.

Overrepresentation of rare genetic blood diseases

This concentration had the effect, among other things, of promoting a rare genetic blood disease. In the world, only one person in a million suffers from this syndrome, but 500,000 Quebecers are affected, for only eight million citizens. Culturally, it also results in a very close-knit population, that is, "people who stand together," as Quebeckers say.

Another remarkable fact is that while annual population growth in France in the nineteenth century was about 3 per 1,000, in Lac Saint-Jean the rate was 25 per 1,000. In communities closely monitored by the Catholic Church, having large families of six, seven, ten, or twelve children is an obligation to resist English assimilation.

Crucial data for research into genetically transmitted diseases

Many demographers around the world dream of having such accurate genetic data. This is information that can be crucial for scientists working on communicable diseases from generation to generation. By having such information, it becomes easier to understand certain syndromes and even to find appropriate treatments that apply to a specific portion of the population.

@LukeAnderTroc explored the appearance of population structure in Quebec over centuries in using thousands of genomes and millions of genealogical records! Story involves rivers, mountains, Royal hunting grounds, and asteroids.

— Simon Gravel (@SFGravel) May 25, 2023

>> Also listen: Inbreeding: what consequences on health?

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