I. INTRODUCTION* 



Albinism, or the absence of the normal pigmentation of an organism, is a not 

 infrequent phenomenon in both animals and plants, yet its occurrence in nature is 

 sporadic and has usually been interpreted as an indication of organic weakness. But 

 this interpretation is probably erroneous, for albino races of domesticated animals 

 are apparently not inferior in vigor to other races. Such is demonstrably the case 

 with albino mice. 



The idea that albinos lack constitutional vigor may have its origin in the ob- 

 servation that, in crosses between albinos and normal individuals, no albino offspring 

 are produced. But the disappearance of albinism in this case is not final. The 

 albino character has not ceased to exist, but has merely become latent in the off- 

 spring. It will reappear unimpaired in the next generation if the cross-bred individ- 

 uals be mated inter se. 



The disappearance of the albino character for a generation, and its subsequent 

 reappearance under close breeding, show that it is inherited in conformity with Men- 

 del's law of heredity, and that it is, in the terminology of that law, a recessive 

 character. 



II. MENDEL'S PRINCIPLES OF HEREDITY. 



The Mendelian principles of heredity, stated in the phraseology of present-day 

 biology, are as follows: 



1. Every gamete (egg or spermatozoon) bears the determinants of a complete 

 set of somatic characters of the species. When two gametes meet in fertilization, 

 there are accordingly present in the fertilized egg the representatives of two sets of 

 somatic characters, which may or may not be the same. If they are the same for 

 a given character, as, for example, coat-color in mammals, the individual which 

 develops from the egg must inevitably have that same character. Thus, when gametes 



* This paper contains a preliminary statement of certain results of breeding experiments with mice, guinea- 

 pigs, and rabbits, which have been conducted in the Zoological Laboratory of Harvard University during the last 

 two and a half years. The experiments with mice are the work principally of the junior author; those with guinea- 

 pigs and rabbits, of the senior author. 



381 



