CASTLE. — LAWS OF HEREDITY. 229 



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 

 (heterozygote, Bateson, :0 ) 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, in respect to any particular character, like those 

 which united to produce it, — half like the maternal and half like the pater- 

 nal 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 conclu- 

 sively 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 ex- 

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

 supposed, 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, gametes W and W 



And the possible combinations of these 2 sets are their product 2 G W + 2 WW 



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 placing the (invisible) W within a parenthesis. 

 Further, in the expression 2 WW one of the identical letters may be 

 dropped as superfluous. Our formula, representing the outcome of the 

 breeding test described, then reads 2 G ( W) + 2 W, and signifies that 

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

 the remaining two white. 2) When two cross-bred (or hybrid) gray 

 mice are bred together, the offspring consist of gray mice and white 

 mice in the ratio of three gray to one white. Moreover, breeding tests 

 show that of the three gray mice thus obtained one is pure, that is, will 

 form only gametes bearing the gray character, while tw T o are hybrid, that 

 is, will form gametes some of which bear the gray character, others the 

 white character. This is precisely the result expected under our hypoth- 

 esis that each hybrid individual forms gametes G and Win equal numbers. 



