394 MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 



is probable that particular chromosomes, or part chromosomes, in the mosaic germ, 

 contain the dominant character, while the remaining chromosomes, or part chromo- 

 somes, contain the recessive character. 



VII. CROSS-BREEDING, REVERSION, AND THE DOCTRINE OF 



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 correct, the cross- 

 bred should form gametes DR 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 reces- 

 sives. 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 DR 

 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 experiments show the formation at the second filial genera- 

 tion 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.* 



Yet we must not fail to observe that a cross of the sort just described is not with- 

 out its effects on the nature of the gametes ; these do not retain their original character. 

 For whereas the mosaic gametes of the original spotted parents produced, on union 

 in pairs, invariably black-white offspring, the mosaic gametes formed by their hybrid 

 offspring, when similarly combined, formed in von Guaita's experiments eight gray- 

 white offspring, but only one black-white. Accordingly, though it seems certain that 

 mosaic gametes may in crosses retain their mosaic character, the cross is nevertheless 



* Compare the results of Darbishire, as stated on p. 390. 



