CASTLE AND ALLEN. — THE HEREDITY OP ALBINISM. 617 



somewhat similarly marked Holstein and Hereford cattle, though they 

 do not breed so true inter se as dark-pointed albino guiuea-piss or 

 Himalayan rabbits or, perhaps, as pink-eyed dancing mice, nevertheless 

 indicate a fairly precise localization of the pigment-forming and albino 

 characters within mosaic germs. 



If we adopt the Roux-Weismannian idea of the nature of the chromo- 

 somes, it is probable that particular chromosomes, or part chromosomes, 

 in the mosaic germ, contain the dominant character, while the remaining 

 chromosomes, or part chromosomes, contain the recessive character. 



VI. Cross-breeding, Reversion, and the Doctrine op 



Gametic Purity. 



Union with a recessive gamete usually, though not always, serves to 

 break up this localization, allowing the dominant character to extend its 

 influence throughout the entire body. This is the case, for example, in 

 the cross between spotted and white mice in the experiments of Haacke, 

 von Guaita, and in part of Darbishire, as well as in our own. It is 

 possible to suppose in such cases either (1) that the resolving effect of the 

 cross is restricted to the soma of the cross-bred, or (2) that it extends 

 also to the germ-cells of the cross-bred. If the former hypothesis is cor- 

 rect, the cross-bred should form gametes D R and R in equal numbers ; 

 if the latter, then only gametes D and R should be formed, and these in 

 equal numbers. A simple test is afforded by the breeding inter se of 

 hybrid mice produced by crossing pure mosaics with recessives. On the 

 first hypothesis suggested, the offspring of the hybrids should consist, in 

 at least one case out of four, of spotted mice formed by the union of two 

 pure D R gametes ; on the second hypothesis no spotted mice should be 

 produced, but only classes D, D{R), and R, as in breeding together 

 hybrids between wild gray mice and white mice. Von Guaita's experi- 

 ments show the formation at the second filial generation of nine spotted 

 mice, twenty-one uniformly gray or black mice, and fourteen white mice. 

 The expectation on hypothesis (1) is 11 spotted: 22 gray or black: 11 

 white mice, which approximates closely the observed result; whereas on 

 hypothesis (2) there should be no spotted mice, but only such as are 

 pigmented all over or else are albinos. The result is conclusive in favor 

 of hypothesis (1) — that a mosaic gamete, on uniting in fertilization with 

 a recessive gamete, does not lose its own identity nor undergo resolution 

 into its constituent parts.* 



* Compare tlio results of Darbishire, as stateil on i>. 012, fuotiiote. 



