CASTLE AND ALLEN. THE HEREDITY OF ALBINISM. 611 



IV. Pure and Hybrid Mosaics. 



In our experiments in crossing spotted with albino mice, a third black- 

 White male was employed. He was a half-brother* to the two spotted 

 males previously mentioned, born of the same mother but by a different 

 sire. The white areas on his body were less extensive than those on 

 his two brothers. He was bred to the same stock of white females as 

 they, but with different results. By him the albinos bore albino 

 as well as pigmented offspring ; of the former twenty-one, of the latter 

 twelve, ten being gray and two black. The albino offspring were found 

 in this case, as in all others tested, to breed true inter se. 



It is evident that the third black-white male differed in nature from 

 his two half-brothers, for he formed recessive gametes, whereas they 

 did not. 



Examination of other breeding records of spotted mice kept by us 

 during the past two years shows that it is possible in these also to dis- 

 tinguish two different sorts of mosaic individuals. These are : — 



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

 They bred 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, D R and R respectively. Accordingly, when 

 bred inter se, they produce offspring of three different sorts, D R, 

 D R- (R) [or D(R) • (R)"],f and R, that is, pure mosaics, hybrid mosaics, 

 and recessives. Theoretically these three classes of offspring should be 

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

 are approximated 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 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 



* In an earlier paper (Castle, : 03 , p. 542) it is stated erroneously that this 

 mouse was an own brother to the other two. 



t 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 R. 



