THE 



AMERICAN NATURALIST 



Vol. XLVII January, 1.913 No. 553 



FACTOES AND UNIT CHARACTEES IN MEN- 

 DELIAN HEEEDITY 



PROFESSOR T. H. MORGAN 

 Columbia University 



The factorial hypothesis has played an important role 

 in Mendelian heredity, and while students of Mendel's 

 principles have had on the whole a pretty clear idea of 

 the sense or senses in which they have made use of factors 

 or symbols, yet those not engaged in the immediate work 

 itself have, I believe, often been misled in regard to the 

 meaning attached to the term factor, and by the assumed 

 relation between a factor and a unit character. The con- 

 fusion is due to a tendency, sometimes unintentional, to 

 speak of a unit character as the product of a particular 

 unit factor acting alone, but this identification has no real 

 basis. It has, in fact, more than once been repudiated, 

 yet the confusion has been so persistent that I venture to 

 try to make clear my own position at least— it is one I 

 think with which in the main many students of heredity 

 will agree — in regard to the relation between unit-factors 

 and unit-characters. I shall do this by means of several 

 examples taken from my breeding experiments with the 

 fly, Drosophila ampelophila. 



The eye of this fly is red. A mutant arose with a ver- 

 milion eye. Crossed to the wild or red-eyed fly, the new 

 color proved to be a Mendelian recessive. 



