Two genetically distinct forms of bipolar affective disorder?

In five pedigrees in which bipolar affective illness (BPD) can be traced through three generations with no father-son transmission, no linkage to HLA could be established. This group was evidently dependent on a single dominant gene (X-linked?), for which the noncommittal designation BPD1 was proposed. Two pedigrees of BPD, in which father-son transmission occurred at least twice, showed highly suggestive linkage to HLA (lod = 2.61 at theta = 0.15). BPD2 was proposed as the designation for this gene. Loss of BPD in one HLA-A/B cross-over suggests that a single dominant gene in this group has a locus in p2-pter region of chromosome 6.