Mendelian Factor Differences Versus Reaction System Contrasts in Heredity. II

Abstract

Summarizing briefly the content of this paper, the following facts have, on the experimental side, been presented with reference to a species hybrid: 1. N. sylvestris when crossed with various varieties of N. Tabacum gives F1 hybrids which are replicas on a large scale of the particular Tabacum variety concerned in the cross. 2. The F1 hybrids of sylvestris and Tabacum produce a small number of functional ovules which represent the sylvestris and Tabacum extremes of a recombination series, the great majority of the members of which fail to function because of mutual incompatibility of the elements of the two systems. 3. Back crosses with sylvestris give sylvestris and aberrant forms, and of the two the sylvestris alone are fertile and breed true. On the other hand, back crosses with Tabacum produce apparently only Tabacum forms of which some are completely fertile and continue to produce only Tabacum forms. On the theoretical side the following conclusions have been drawn and their application indicated: 1. As a consequence of modern Mendelian developments, the Mendelian factors may be considered as making up a reaction system the elements of which exhibit more or less specific relations to one another. 2. Strictly Mendelian results are to be expected only when the contrast is between factor differences within a common Mendelian reaction system as is ordinarily the ease in varietal hybrids. 3. When distinct reaction systems are involved, as in species crosses, the phenomena must be viewed in the light of a contrast between systems rather than between specific factor differences, and the results obtained will depend upon the degree of mutual compatibility displayed between the specific elements of the two systems. 4. Sterility in such eases depends upon non-specific incompatibility displayed between the elements of the systems involved, and the degree of this sterility depends upon the degree of such incompatibility rather than upon a certain number of factors concerned in the expression of such behavior. 5. The consequences of the application of such a conception to the complex type of behavior in (Oenothera are pointed out, and the suggestion is specifically made that the type of behavior exhibited by Lamarckiana and its segregants in hybridization may be referred to such complex system interactions.