MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 391 



The idea that there are, as just explained, two different sorts of mosaic indi- 

 viduals originated with the junior author of this paper. It constitutes a discovery 

 of no small importance, one which lends strong support to the Mendelian hypothesis 

 of essential gametic purity. It shows that alternative parental characters, when 

 united in fertilization, do not mix, but that each retains its own identity and sub- 

 sequently separates from the other when gametes are formed. This takes place even 

 when one of the parental characters is itself a mosaic ! Herein we have a confirmation 

 of the conclusion based upon morphological observations, that the paternal and 

 maternal contributions to the zygote retain each a distinct individuality. Further, 

 the idea of Bateson receives confirmation, that the gray color of mice obtained by 

 crossing black-white with white mice is itself a " heterozygote " character. 



VI. PURE AND IMPURE RECESSIVES. 



Although certain of Darbishire's conclusions are, as we have seen, untenable, 

 he has made observations of great significance, one of which deserves particular notice. 

 In his crosses with dancing mice he employed two different stocks of white mice, 

 and with results somewhat different in the two cases. The two stocks were (1) "pure- 

 bred" albinos purchased from breeders, and presumably descended from white parents, 

 and (2) "cross-bred" albinos known to be descended from spotted (i.e., hybrid mosaic) 

 parents. The latter when crossed with the fawn-white dancing mice produced 

 twenty-five offspring spotted with white, and only four which were pigmented all 

 over. The "pure-bred" albinos when similarly mated produced thirty-six spotted 

 mice and sixty-one mice pigmented all over. In other words, the cross with "pure- 

 bred " albinos produced a much smaller proportion of spotted mice than did that with 

 "cross-bred" albinos. 



Apparently the primary effect of a cross between a mosaic and an albino individual 

 is to release in the soma of the offspring the dominant character either latent or tied 

 up in a firm mosaic with the recessive character in the gametes of the spotted parent. 

 The dominant character is thus free to extend its influence over the entire soma of 

 the hybrid offspring. This release seems to be the more complete the more free 

 the albino stock has been kept from crosses with pigmented animals. 



It follows from Darbishire's observation that not all albinos breed alike when 

 crossed with the same pigmented stock, a conclusion which our own experiments fully 

 substantiate. This indicates that the gametes formed by different albino stocks are not 



