Abstract
In a sample of 51 like‐sex DZ twin pairs (22 male, 29 female) 49 quantitative traits were analyzed for association with, and linkage to, each of 7 informative blood systems. Nine associations were found to be significant at the 0.01 level, and a further 27 significant at the 0.05 level, whereas the numbers expected to be significant by chance alone are about 3 and 17 respectively. The most consistent associations are those of Rh with the scores obtained with Paul Griéger's characterological test. The associations may be due to a factor present in the physical or social environment that influences both characteristics simultaneously; alternative explanations are pleiotropy, epistasis, or heterogeneity of the populations. Only 12 of the tests for linkage were significant at the 0.05 level, less than would be expected by chance alone; these results are therefore inconclusive. The most significant linkage is that between Hp and activity, one of the 3 scores in Griéger's characterological test. In vew of independent data suggesting evidence of linkage between Hp and depression spectrum disease, further study of this possible linkage in larger families would be warranted.