To the Editor.—
I have been greatly puzzled over the discrepancy between the 1972 and 1974 pedigree series of Mendlewicz and colleagues1,2 and the findings of myself and my colleagues3 that do not replicate the earlier findings. The Mendlewicz et al series clearly shows tight linkage between bipolar and related affective illness and the color blindness region on the X chromosome, and our series just as clearly rules out this linkage. Both series were collected in essentially the same population, 200 miles apart. Each series is homogeneous (one for linkage and one for nonlinkage) by Morton's test for homogeneity of lod scores (a linkage measure).4 Mendlewicz et al have recently suggested5 that there is genetic heterogeneity since his recent series has only one (of eight) pedigrees that strongly favors linkage, and this series is heterogeneous by Morton's test. But why, then, was his earlier series homogeneous?