Duplicate genes for capsule-form 

 in Bursa bursa-pastoris^. 



By George Harrison Shull, 



Station for Experimental Evolution, Cold Spring Harbor, Long Island, New York. 

 (Eingegangen am 16. Januar 1914.) 



The important discovery of NILSSON-EHLE (1908, 1909) and 

 EAST (1910) that the same apparent or "phenotypic" characteristic 

 may be produced independently by any one of several Mendelian 

 factors which are not allelomorphic to each other, causing the alter- 

 native characters (when dominance is complete) to segregate in the Fa 

 in the ratios 15 : 1, 63 : 1, 255 : 1, etc., has opened the way to a 

 Mendelian interpretation of several classes of phenomena which have 

 been rather generally regarded as non-Mendelian , including the inheri- 

 tance of apparently continuous quantitative differences, the apparent 

 modification of unit-characters by selection, the apparent failure to segre- 

 gate in Fa, and the occasional appearance of atavistic or otherwise 

 aberrant individuals which, because of their recessive character, breed 

 true, and which in consequence have been generally considered mutants. 



As these phenomena are in the aggregate of very frequent occur- 

 rence, the availability of this Mendelian interpretation for any consid- 

 erable portion of them must depend upon the correctness of the 

 assumption that such duplication of determiners is also frequent. 

 While this may not seem inherently improbable, the number of fully 

 demonstrated cases of this kind can still be counted on the fingers 

 of one hand, and it becomes a matter of considerable importance to 

 add to this list. 



To account for certain ratios which have appeared in the hybrids 

 between Bursa bursa-pastoris and B. Heegeri in respect to the form of 



x ) Read before the American Society of Naturalists in Cleveland, Ohio, January 2, 

 1913. Extended to include discussions of more recent literature. 



