SPECIFICITY OF GENE EFFECTS 243 



no matter in what gene complex they may occur. That is, other genes than 

 the causative ones have no measurable influence upon their expression. A 

 possible exception to this statement might be proposed for the A and N char- 

 acters, res])ectively, since each is somewhat less readily agglutinated when in 

 the heterozygote, AB and MN, than when either occurs singly. 



THE HYBRID SUBSTANCE IN SPECIES HYBRIDS 

 Until the early part of this century, most of the workers in immunology 

 had reached the conclusion that the specificities obtained in immunological 

 reactions were primarily if not entirely concerned with proteins. Therefore, 

 the finding by Heidelberger and Avery (1923, 1924) that the immunological 

 specificities of the pneumococcal types were dependent upon polysaccharides 

 was indeed a forward step in our understanding of the chemical nature of 

 biological specificity. It is a pleasure to acknowledge that this work of 

 Heidelberger and Avery convinced the writer that immunological technics 

 should be a useful tool in studying genetic phenomena. Also, although at that 

 time pollen differing in gene content seemed (and still does) to be promising 

 experimental material, the species and species hybrids in pigeons and doves 

 produced by the late L. J. Cole were tailor-made for further studies. 



Pigeon-Dove Hybrids 



The first step was to determine whether the cells of one species could be 

 distinguished from those of the other. In brief, all the comparisons by im- 

 munological technics, between any pair of species of pigeons and doves, have 

 resulted in the ability to distinguish the cells of any species from those of an- 

 other, and to show that each species possessed antigenic substances in com- 

 mon with another species, as well as those peculiar to itself — those species 

 specific. A dozen or more kinds of species hybrids have been obtained in the 

 laboratory, and in general, each kind of hybrid has contained in its cells all 

 or nearly all of the cellular substances of both parental species. One such 

 species hybrid is that obtained from a mating between males of an Asiatic 

 species, the Pearlneck (Slreplopelia chinensis) and the domesticated Ring 

 dove females {St. risoria). The corpuscles of these hybrids contained all the 

 substances common to each parental species, but did not contain quite all the 

 specific substances of either parental species. Further, the cells of these 

 hybrids did possess a complex of antigenic substances not found in the cells 

 of the parents. These relationships are presented in Table 15.1 and are given 

 in diagrammatic form in Figure 15.1. This new antigen has been called the 

 "hybrid substance," and it has been ])resent in every hybrid produced be- 

 tween these two species. 



Upon repeatedly backcrossing these species hybrids and selected back- 

 cross hybrids to Ring dove, ten antigenic substances which differentiate 

 Pearlneck from Ring dove have been isolated as probable units. That is, a 



