Commingling and segregation analysis of blood pressure in a French-Canadian population

Commingling and segregation of age-sex-adjusted systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial blood pressure (MBP) were examined in 1,560 individuals from 374 French-Canadian nuclear families. After correction for skewness, evidence in favor of two commingled distributions was found for SBP in the combined data (parents and offspring) and in parents, but not in offspring. Segregation analysis (using the computer program POINTER) suggested that a multifactorial contribution to all three phenotypes was greater in offspring than in parents, which could be the result of either polygenic or shared environmental components relevant to sibships, or both. Statistical evidence was found for a major effect on SBP. However, Mendelian transmission of the major effect was rejected, and no transmission of the major effect (equal τ's) was not. This is just the opposite to what would be expected if the major effect was due to a major gene, and it would ordinarily be considered as sufficient evidence to refute a major gene effect on SBP. However, the commingling in parents but not in offspring (who are all below 26 years of age), and the finding of equal transmission probabilities (nearly equal to 1), are compatible with an alternative interpretation. It is possible that there is a real major gene effect on SBP but that the genotype for elevated SBP has not yet expressed itself in the offspring as they have not yet gone through the risk period. Accordingly, this possibility needs to be evaluated further in additional studies involving older offspring.