Duplicate genes for capsule-foini in Bursa bursa-pastoris. \ \ 7 



obviously corroct inference were it not for the fiiet, already inentiened. 

 that our cultural treatment of Bursa has become much more successful. 

 Elimination after germination has been almost completely conquered; 

 at any rate it has been reduced until it has become a nearly negligible 

 factor. The fact that these later generations have given a nearly 

 normal distribution of the ratios uuiy consequently be taken as strongly 

 supporting the hypothesis that the deficiency of Heegeri plants in the 

 earlier cultures was due to selective elimination. The cause for the 

 elimination was probably not, however, as was assumed, the constitu- 

 tional weakness of the Hacgcri type, but only its longer period of vege- 

 tative development. There has been in general an excess of the biima- 

 pastoris type among the ])lants first coming t(i bloom and a corresponding 

 excess of Heegeri among the plants last to bloom. Tliis i-elation will 

 be considered moi-e in detail on another occasion. It need only be 

 pointed out here that if cultures which have stood for a long tinie in 

 the greenhouse become unhealthy from the attacks of insects or fungous 

 pests, or from the cumulative effects of any unfavorable conditions of 

 the environment, the more slowly developing plants will suffer most; 

 and if many die from such causes without having fruited, there would 

 result just the differential elimination necessary to explain the deficiency 

 of Heegeri plants in the injured families. 



While at first sight this seems to be an adequate explanation of 

 the deficient I'atios, there are indications that this may not be the whole 

 story. The elimination of a disproportionate number of Heegeri plants 

 should have no influence on the composition of the bursa-pastoris portion 

 of the same family. In the case of a monohybrid family this would 

 mean that one-third of the bursa-pastoris plants would be homozygous 

 and two-thirds heterozygous, regardless of the deficiency in the number 

 of recessives present. The only monohybrid family (No. 092841 in the 

 F3 from the original cross, consisted of 42 bursa-pastoris and 9 Heegeri 

 or 4-67 : 1. An attempt to test the constitution of all the bursa-pastoris 

 individuals in this family by selfing them, was successful in the case 

 of 39 of them, and these 39 wer£ shown by their progenies (Table IV) 

 to have consisted of 8 homozygotes and 31 heterozygotes, instead of 

 the 13 homozygotes and 26 heterozygotes that were to be expected. 

 It is thus seen that this F3 family (No. 09284) really presented a ratio 

 approximately 1:4:1 instead of 1:2:1. 



The fact that the deficiency in the recessive class is balanced 

 by a similar deficiency in the number of homozygous dominants may 

 be merely a coincidence, of course, this possibility being rendered 



