390 MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 



1. Pure mosaics, spotted and forming only mosaic gametes, DR. They breed 

 true inter se, but when crossed with albinos produce only individuals of the following 

 class. 



2. Hybrid mosaics, usually more extensively pigmented than pure mosaics, often 

 pigmented all over. (Compare the results of Haacke, von Guaita, and Darbishire.) 

 They form, in approximately equal numbers, mosaic and recessive gametes, DR and 

 R, respectively. Accordingly, when bred inter se, they produce offspring of three 

 different sorts, DR, DR-(R) [or D(R) ■ (R)],* and R, that is, pure mosaics, hybrid 

 mosaics, andf recessives. Theoretically these three classes of offspring should be 

 numerically as 1 : 2 : 1. There is reason to believe that these proportions are approxi- 

 mated in our experiments, but this matter has not yet been fully tested. When 

 bred to albinos, hybrid mosaics produce (in equal numbers) hybrid mosaic and 

 albino offspring. 



It is possible to recognize in the experiments of von Guaita and Darbishire also 

 these two different classes of mosaics. Von Guaita's original stock of dancing mice 

 consisted of pure mosaics, for they bred true inter se, and when bred to albinos pro- 

 duced only pigmented offspring. These offspring, though not themselves spotted, 

 were clearly hybrid mosaics, for when bred inter se they produced spotted as well as 

 albino offspring, of the former nine, of the latter fourteen, in a total of forty-four 

 young; the expectation on our hypothesis is eleven of each! 



The evidence afforded by Darbishire's mice is even more convincing because 

 of a visible difference between the heterozygous (hybrid mosaic) form and both of 

 the pure parental forms. The parents were, as we have seen, in every case pink- 

 eyed, whereas the hybrids were as invariably dark-eyed. Hybrids bred inter se should 

 in this case, according to our hypothesis, produce pure mosaics (pink-eyed), hybrid 

 mosaics (dark-eyed), and albinos, in the proportions, 1:2: 1. These three sorts of 

 individuals were actually observed by Darbishire, and in very nearly the theoretical 

 proportions. In a total of sixty-six young the observed numbers are 17 pink-eyed 

 mosaics : 36 dark-eyed individuals : 13 albinos; the expectation is 16.5 : 33 : 16.5. 

 By crossing hybrids with albinos Darbishire obtained, as we should expect on the 

 hypothesis stated, only two of the three classes produced by intercrossing hybrids, 

 namely, dark-eyed individuals (hybrid mosaics) and albinos; no pink-eyed pigmented 

 individuals were produced. The expectation is that the two classes formed will in 

 this case be approximately equal; the observed numbers are 94 dark-eyed individuals, 

 111 albinos. 



* The period is used to indicate the distinctness, in the zygote, of the gametes which united to form that zygote, 

 as well as to show that, when in the hybrid individual segregation of characters takes place at gamete-formation, 

 splitting will occur at the point marked by the period, producing gametes DR and jB. 



