Some rare inherited disorders are found with a high frequency in the population of the Saguenay region (Québec province, Canada, population 300,000). Inbreeding coefficients are too low to be an explanation for these high frequencies. In the first decades of settlement (1842-1870), most of the immigrants came from a small region called Charlevoix (east of Québec city). As most of the genetic disorders found in the Saguenay region are also found in the Charlevoix region, it is strongly suspected that the genes were introduced by these first settlers from Charlevoix. Using the BALSAC database (which contains linked information on the entire Saguenay population) to calculate the number of contemporary descendants and the genetic contribution of each founder to these descendants, we show that: (1) Founders who entered the population before 1870 contribute to 45% of the contemporary gene pool, despite the fact that they represent only 15% of all the 20,012 immigrants with descendants in the contemporary population (individuals born between 1950 and 1971). (2) Their genetic contribution is not homogenous in the contemporary population: 5% of the population have 100% of their gene pool coming from these first founders, while 10% have a zero to 5% contribution from these earliest founders. (3) Fifty percent of the genes introduced in the population were lost. (4) If only 68 immigrants among the first founders (2.0%) were carriers of the same gene, it could reach a frequency of 5% in the contemporary population, which is the frequency of most of the inherited disorders found in the population.