118 Shull. 



the more probable owing to the small size of the family. If not a 

 mere coincidence, the ratio 1:4:1 suggests the possible correctness 

 of my second original hypothesis, namely, that there may have been 

 here a case of differential mating in which the union of unlike 

 gametes was favored. If this were correct, the evidence from this 

 one family would indicate that a sperm carrying the bursa-pastoris 

 character is twice as likely to fertilize an egg lacking that character, 

 as an egg having it, and vice versa, that sperms bearing only the 

 Hecgeri determiners will fertilize twice as many bursa-pastoris eggs as 

 Heegeri eggs. There is one serious obstacle in the way of this expla- 

 nation of the ratio 1:4:1; among the numerous hybrid families in 

 F 2 there were a number of families in which the excess of Heegeri 

 individuals beyond the expected 25 per cent was quite as striking as 

 was their deficiency in the families of the second and third generations. 

 An attempt is being made to analyze the dominant groups of some 

 of these families in order to determine whether in those cases in which 

 there was an excess of Heegeri, there was also a corresponding excess of 

 homozygous dominants. 



The same fact also stands in the way of another possible inter- 

 pretation of the 1:4:1 ratio: If the heterozygous class were always 

 in excess of expectation, it might be assumed to be due to the physio- 

 logical superiority of the heterozygous individuals over the homozygous, 

 owing to the stimulating effect of heterozygosis. - - a phenomenon now 

 generally recognized. The homozygotes, being weaker, would be elimi- 

 nated in greater measure than the stronger heterozygotes, and it would 

 not be strange that an elimination on this basis should be about equal 

 in both positive and negative homozygotes. Also against such a hypoth- 

 esis, however, is the fact that Bursa is normally self-fertilizing, and 

 that the hybrids are consequently not as a rule markedly superior to 

 the pure types; it is difficult to conceive of their elimination to the 

 extent required by the ratio 1:4:1. 



There remains a fourth possibility, namely, that there is some degree 

 of "linkage" of determiners, resulting in partial "coupling" in some 

 families, balanced by a corresponding "repulsion" in other families. All 

 of these hypotheses need further experimental study, and further dis- 

 cussion of them will be postponed until more data is at hand. 



The phenomenon of plurality of genes having a similar function, 

 i.e., independently producing the same character, is called by LANG- 

 (1911) "polymery" and by PLATE (1913) "homomery". JOHANNSEN 

 (1913) suggests that both these terms be retained, the former for the 



