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  • Update to Heyer's "One Founder/One Gene Hypothesis in a New Expanding Population" (1999)
  • Evelyne Heyer and Frederic Austerlitz
Keywords

Saguenay Population (Quebec), Recessive Disorders, Cultural Transmission of Fertility, Genetic Drift, Linkage Disequilibrium, Mutation.

"One Founder/One Gene Hypothesis in a New Expanding Population" (Heyer 1999) was written as part of a more global project aiming to understand the processes by which several inherited recessive disorders reached a high frequency in the Saguenay region of Quebec. In the paper Heyer focused on a specific question: Was it likely that each disorder had been introduced in the ancestral population by a single founder? The Saguenay population was known for a long time for having been under strong founder effect (Bouchard and DeBraekeleer 1991). Indeed, approximately 5,000 founders who settled in "nouvelle-France" in the 17th century are the ancestors of more than 300,000 individuals living nowadays in the Saguenay region (Heyer and Tremblay 1995). This is a tremendous population growth in only 10-12 generations. The question remained whether this strong increase alone could explain the high occurrence of severe genetic disorders in this population.

Under the one founder/one gene hypothesis the increase in carrier frequency is from 1/5,000 to 1/25 (average present carrier frequency of most common recessive disorders). This represents a strong level of genetic drift, which is quite unlikely in a standard Wright-Fisher population (i.e., a population of neutral genes without specific demographic processes). Using ascending genealogies in the Human Biology paper, Heyer proved that such an increase was expected from the genealogical network of the population, confirming the one founder/one gene hypothesis.

The next step of the work was to find the demographic process that could explain this tremendously strong increase of frequency of the disease alleles. Heyer showed the existence of a peculiar sociodemographic process: the inheritance of fertility through cultural transmission. This was evidenced by a positive correlation between the number of progeny of an individual and the number of progeny of his or her parents. Heyer computed this correlation using the demographic database built up by the Interdisciplinary Research Group on Demography and [End Page 657] Genetic Epidemiology (http://www.uqac.ca/grig), which includes all the demographic events and the genealogical links between individuals from the Saguenay population for more than 10 generations. Basically, individuals belonging to large sibships were more likely to produce numerous offspring than individuals with few siblings. More specifically, in the Saguenay population, what was transmitted was the number of effective children, that is, the number of children who settled and had themselves at least one child in the population (Heyer and Cazes 1999). The total number of children, which also includes children who settled outside the population and children who did not reproduce, was much less transmitted from one generation to the next.

Austerlitz and Heyer (1998) showed through a simulation study that this cultural transmission of fertility made possible the strong increase in frequency of inherited disorders observed in the Saguenay population. It also decreased by a large amount the effective population size of the population, from the 17,000 calculated based on demographic growth to only 900 when fertility transmission was included. Indeed, despite fast population growth, the effective population size decreased through time (Mourali-Chebil and Heyer 2006)! This decrease in population size can be explained by the fertility transmission, the effect of which cumulates through generations, therefore increasing drift over time. The fertility transmission detected in the demographic-genealogical database has also been studied for the whole Quebec population for the first two centuries of settlement (Gagnon and Heyer 2001). Fertility transmission speeds up the genetic differentiation between regions (Gagnon et al. 2006).

Fertility transmission is a clear example where our strong capacity as humans to transmit not only genes but also culture has an impact on our genetic evolution. This interaction between cultural forces and genetic evolution has been one of the major focuses of our research since then.

In short, using demographic data, we were able to measure the cultural transmission of demographic behavior from one generation to the next and to evaluate the effect of this transmission on the evolution of...

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