382 MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 



formed by one white mouse, meet in fertilization gametes formed by another white 

 mouse, the offspring are invariably white. Similarly, when a wild gray mouse is bred 

 to another wild gray mouse, the offspring are invariably gray. And when a pure- 

 bred spotted black-white mouse is bred to a mouse like itself, the offspring are all 

 spotted black-white. 



2. But when the two gametes uniting bear each what represents a different 

 somatic character, only one of these characters may be manifested by the individual 

 (or zygote) formed. Thus, when wild gray mice are mated with white mice, only gray 

 offspring are produced. The gray character is, in Mendel's terminology, dominant; 

 the white character, recessive. Or, when wild gray rats are mated with black- white 

 rats, only gray rats are produced. The wild gray character is, accordingly, dominant 

 not only over white, but also over black- white. 



3. Sometimes the zygote formed by the union of two unlike gametes (hetero- 

 zygote, Bateson, : 02) develops the character of neither parent in its purity. It may 

 have a character intermediate between those of its parents, or something entirely 

 different from either. Thus, when black-white mice are mated with white mice, the 

 offspring are gray like the wild house-mouse. 



4. Whatever the somatic character of the zygote is, the germ-cells which it forms 

 will be, like those which united to produce it, half like the maternal and half like the 

 paternal gamete. Thus a gray mouse obtained by crossing a wild gray mouse with 

 a white one forms in equal numbers gametes which bear the gray character and those 

 which bear the white character. This is conclusively shown by two simple breeding 

 tests : (1) When a cross-bred (or hybrid) gray mouse is bred to a white mouse half 

 the offspring are hybrid grays, half are white. This is precisely the result we should 

 expect if the cross-bred gray mouse forms, as we have supposed, in equal numbers, 

 gametes which bear the gray and those which bear the white character. For the 

 gray mouse will produce 



gametes G and W; 



the white mouse forms gametes W and W; 



the possible combinations of these two sets are 2GW+2WW. 



But, as we have already stated, when a zygote contains both the gray character and 

 the white character, only the former will be visible. This may be indicated by plac- 

 ing the W within a parenthesis. Further, in the expression 2 WW, one of the identical 

 letters may be dropped as superfluous. Our formula then reads 2 G(W) +2W, and 

 signifies that two in every four of the offspring produced will be gray hybrids, and 



