Academic journal article Population

The Transmission Probabilities of Founder Genes in Five Regional Populations of Quebec

Academic journal article Population

The Transmission Probabilities of Founder Genes in Five Regional Populations of Quebec

Article excerpt

Population genetics studies seek to determine the magnitude of genetic variability in natural populations and to explain its origin and persistence and its relevance to the evolution of populations (Hartl, 1984). When a mutation occurs in any given gene, leading to a malfunction of this gene and a risk of disease, the diffusion or disappearance of the mutated gene in the population is determined primarily by the demographic behaviour of the carriers. Although new mutations are rare, all individuals carry abnormal genes (between four and five lethal recessive genes) among the genes present in their cells (Vogel and Motulsky, 1986).

Several studies have shown certain rare hereditary disorders to have a particularly high incidence in Quebec (Bouchard and De Braekeleer, 1991 and 1992; Vézina, 1996; Scriver, 2001). Conversely, other disorders commonly observed in European populations are practically non-existent in some regions of Quebec (Bouchard and De Braekeleer, 1992). The high (or low) carrier frequencies of deleterious genes can be explained by founder effects or by the phenomenon of genetic drift. A founder effect occurs when a new population is formed from a relatively small number of individuals belonging to the same parent population (Bouchard et al., 1991a; O Brien et al., 1994). Genetic drift is a modification of genie frequencies in a population resulting from the random hazards of transmission (Hartl, 1994). In the case of founder effects, people with hereditary disorders have a very high probability of carrying the same mutation. It thus becomes possible to develop tests for carriers in order to improve detection and prevention of these disorders. The high frequencies of hereditary disorders observed in Quebec are generally attributed to one or a combination of several founder effects.

These diseases are not evenly distributed across the various regions of Quebec (Table 1). Certain genopathies are observed mainly in the north-east regions of the province (notably those of Saguenay and Charlevoix), while others are more frequent in the south and the west. These differences derive largely from founder effects and from regional variation in migration patterns; in this study, we have attempted to bring some of these effects into sharper focus. Using the method of gene dropping on reconstructed genealogies, we have measured and compared the transmission of founder genes in five regional populations of Quebec.

I. Data and methods

1. Regions studied and genealogical data

The data used come from genealogies reconstructed with information about 756 individuals who were married in five regions of Quebec (Beauce, Terrebonne, Charlevoix, Rimouski and Saguenay) between 1900 and 1971 (Table 2). These regions were chosen for their distribution across the territory of Quebec (Map 1) and for the availability of data at the time the study was conducted. Data from the Charlevoix and Rimouski regions were first used in the study by Tremblay et al. (2000). Heyer et al. (1997) used data from the Saguenay, and Bergeron (2001) used data from the Beauce. Finally, data for the Terrebonne region were compiled specifically for the present study.

With the exception of the Beauce region, all the starting points for the genealogies were taken from the BALSAC Population Register, which contains information derived from marriage, baptism, and burial records (Bouchard et al., 1991b). In the case of the Saguenay and Charlevoix regions, we started with parents of children born between 1950 and 1971 (chosen at random from baptism records). For Rimouski and Terrebonne, marriage records were randomly chosen from those available for each period. For the Beauce region, the initial data come from the marriage records published for this region by the Quebec Genealogy Society (1995); these are marriages that served as control cases for an earlier study (Bergeron, 2001).

The genealogies were reconstructed mainly by means of the BALSAC Population Register and the BALSAC-RETRO Genealogical Database. …

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