Genetic markers in depressive disorders.

1. Association or linkage between a known genetic marker and an illness with a presumed genetic etiology supports two conclusions: a) The genetic etiology would be considered definite, and b) the illness should be considered a homogeneous disease. 2. After separating depressions on the basis of gross familial differences, a finding of linkage or association in any subgroup would indicate that the particular illness is autonomous. Data on association between bipolar and unipolar depression and subtypes of the ABO system are given. 3. Methodological problems in association studies are discussed. 4. Preliminary data suggest the possibility of linkage between the alpha-haptoglobin locus and third component of complement locus and depression spectrum disease, a depression which is familially defined by the presence of alcoholism in the first-degree family member.