Consanguinity and kinship in Down syndrome in Saguenay Lac-Saint-Jean (Québec).

Genealogies of 67 cases of Down syndrome (DS) cytogenetically proven as trisomy 21 born in Saguenay-Lac-Saint-Jean were reconstructed using a population register. Genealogies of three controls for each patient matched on sex, date and place of birth were also reconstructed. The mean kinship coefficient was slightly, but not significantly (p > 0.05), increased in the DS group compared to the control group (0.000222 versus 0.000165). The increase was mostly the result of two patients being related as uncle-nephew. The mean inbreeding coefficient was also higher, although not significantly (p > 0.05), in the DS group than in the control group (0.001348 versus 0.000848). Nine consanguineous marriages (13.4%) were identified in the DS group compared to 17 of 201 (8.5%) in the control group. These results may suggest, as previously reported by Roberts et al. in Shetland, that some recessive element is involved in the etiology of DS, possibly in preventing the loss of the trisomic 21 fetus.