THE INCONSTANCY OF UNIT-CHARACTERS 1 



PROFESSOR W. E. CASTLE 

 Harvard University 



Theee can be no reasonable doubt that Mendel's law 

 is of fundamental importance in genetics. It explains so 

 many of the anomalous facts and seeming contradictions 

 encountered in practical breeding. The basic fact under- 

 lying this law is the existence of unit-characters, inde- 

 pendently inherited. Their independence makes it pos- 

 sible to combine them in any desired way through the 

 agency of cross-breeding. 



In the first flush of enthusiasm over the rediscovery of 

 Mendel's law it was thought by some that recombination 

 of unit-characters through crossing was to solve all the 

 problems of breeding relating to the production of new 

 and improved varieties. But experienced animal breed- 

 ers have, as a rule, been very conservative in their ex- 

 pectations, a conservatism justified by the knowledge of 

 how painfully slow and tedious is the process of improv- 

 ing a breed in any essential regard. For though it is 

 easy enough in two generations to get new color varieties 

 by crossing breeds of different color, the new creations 

 will, in respects other than color, not be the same as 

 either of the breeds crossed ; they may be inferior to both 

 in every respect but color, and it will be a difficult, if not 

 impossible, task to restore the desirable qualities lost. 

 The reason is that our improved breeds differ from each 

 other in so many minor characteristics that it is quite 

 impossible to give attention to all of them simultaneously. 

 For as the number of variable characters resulting from 

 a cross increases, a particular combination of characters 

 will become more rare in occurrence and harder to fix. 



Soon after it was discovered that unit-characters exist, 



*An address delivered at the University of Illinois, April 19, 1912. 

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