Coefficients of relationship by isonymy among the parents of Scottish twins: a test of familial aggregation.

The ability of the coefficient of relationship by isonymy Ri to detect familial aggregation of conditions suspected of being under a degree of genetic control was tested on groups of fathers and mothers of like-sex twins, fathers and mothers of unlike-sex twins, and fathers and mothers of singleton controls born from 1977 to 1981 in Scotland. No statistically significant difference of within-group or between-group Ri was found, either for all surnames or for rare surnames only. However, the overall pattern of results for rare surnames showed a measure of agreement with what is already known of the genetics of twinning. The relatively high within-group Ri for mothers of unlike-sex twins is consistent with a maternal genetic influence on dizygous twinning, whereas the relatively high between-group Ri for fathers with mothers of unlike-sex twins suggests that paternal as well as maternal genes may be involved. Values of Ri for the parents of like-sex twins raise the possibility of a paternal but not a maternal genetic contribution to monozygous twinning.