As Knut Faber pointed out in his Nosography [i], Mendelism contributed much to nosology (the study of the classification of disease) by focusing attention on specific entities. Bacteriology, with its similar emphasis on specific etiology and specific entities, had a comparable effect. One need only recall that a century ago, symptoms such as jaundice, dropsy, and anemia were viewed as entities, in much ofmedicine at least, to realize the nosologic contributions of Mendelism and bacteriology. But the main object here is to review some contemporary problems in the nosology of genetic disease.

The two leading principles in genetic nosology are pleiotropism and genetic heterogeneity." Pleiotropism" means multiple effects of a single etiologic factor, for example, a single gene in the genetical use to which the term is usually put. Pleiotropism is the usual, but not the sole, basis for hereditary syndromes. Linkage, that is close situation on the same chromosome of genes for the separate manifestations, is a theoretically unsatisfactory and as yet unproved explanation for Mendelizing syndromes. Because of crossing over, which in time separates even closely situated loci, linkage produces no permanent association of traits in a population.