ON SUCCESSIVE DUPLICATE MUTATIONS. 1 



R. RUGGLES GATES, 

 UNIVERSITY OF LONDON. 



Nilsson-Ehle 2 was the first to formulate the hypothesis of 

 duplicate factors or representatives for the same character. 

 He brought forward evidence from crosses of red and white 

 varieties in certain Swedish strains of wheat, tending to show 

 that in different F 2 families plants with red and white grains 

 occurred in the ratios respectively 3:1, 15:1 and 63 :i ; from which 

 he concluded that three independent units for red were present, 

 each of which could produce the color alone. Although his 

 conclusions were criticized by Kajanus, 3 yet there remains a strong 

 presumption in their favor, and several other cases of supposed 

 duplicate factors have been described, though these have for 

 the most part rested upon more insecure data than the original 

 instances of Nilsson-Ehle. 



Certain suggestions have been made concerning the origin of 

 this duplicate or triplicate condition. Emerson and East 4 

 pointed out in general terms that if a factor should become 

 located in a different chromosome or should be affected in 

 any way so as not to be always allelomorphic to itself, then a 

 duplication of determiners would result. Shull 5 has listed the 

 supposed cases of duplicate determiners and remarks that such 

 a condition of duplication might also result from "repeated pro- 

 gressive mutations." In the same paper, Shull endeavors to 

 account for the origin simultaneously of a duplicate "gene" 



1 Presented before the American Genetic Association, San Francisco meeting, 

 August 3, 1915. 



2 Nilsson-Ehle, H., 1909, " Kreuzungsuntersuchungen an Hafer und Weizen," 

 I., Lunds Univ. Arsskrift., N.F., Afd. 2, Bd. 5, Nr. 2, pp. 122. 



3 Kajanus, B., 1914, "Zur Kritik des Mendelismus," Zeitschr. f. Abst. u. Vererb., 

 12: 206-224. 



4 Emerson, R. A., and East, E. M., 1913, "The Inheritance of Quantitative 

 Characters in Maize," Agric. Exp. Sta. Nebraska, Research Bull. 2, pp. 120. 



6 Shull, Geo. H., 1914, "Duplicate Genes for Capsule-form in Bursa bursa- 

 pastoris," Zeitschr. f. Abst. u. Vererb., 12; 97-149, Figs. 7. 



204 



