THE DUPLICATION OF A LEAF-LOBE FACTOR IN 

 THE SHEPHERD'S-PURSEi 



GEORGE HARRISON SHULL 



Princeton University 



In two previous papers (Shull, 191 1, 1914) I have demonstrated 

 the existence of two independent MendeHan factors in the shepherd's- 

 purse {Bursa bursa-pastoris) , each of which produces the triangular 

 form of capsule. In the latter paper I discussed at some length some 

 of the criteria and the significance of such "duplicate" factors. I 

 gave also a practically complete list of the relevant literature which 

 had appeared before 1914, and called attention to certain miscon- 

 ceptions which had found expression in a number of the papers cited. 



It is not necessary, therefore, in presenting a new case of dupli- 

 cation of factors in this species, to repeat at any length the discussion 

 in this earlier paper. It is important however to direct attention to 

 the discussions there presented, since several papers along similar 

 lines, or on closely related matters, which have appeared more recently, 

 do not include a reference to my paper, even when from the terminology 

 used it is evident that the authors have had it before them. Several 

 writers are now making the desired distinction between "duplicate" 

 and "plural" factors, and it is to be hoped that in the future, in the 

 interest of precision and accuracy, all those who discuss size-inheritance 

 and related phenomena, will abandon the expression "multiple" 

 factors because of its erroneous implications. 



It has been shown (Shull, 1909, 1910, 191 1) that the form of leaf 

 in shepherd 's-purse is controlled by certain Mendelian genes which 

 have been designated Aa and Bh, the presence of A resulting in an 

 elongation of the primary lobes of the leaf, while the B gene divides 

 the leaf to the midrib, and brings to light certain characteristic second- 

 ary lobing. The several possible combinations of these genes give 

 the four rosette types: AB = heteris, aB = rhomboidea, Ab = tenuis, 

 and ab = simplex. These four forms are illustrated in Figs. 1-4. 



I have now studied the progenies of a considerable number of 

 wild Bursas from places as diverse as Chile, Hawaii, Japan, China, 



1 Contribution from the Station for Experimental Evolution, of the Carnegie 

 Institution of Washinccton, and from the Genetical Laboratory of Princeton Uni- 

 versity. 



427 



