MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 383 



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 

 two 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 hypothesis 

 that each hybrid individual forms gametes, G and W, in equal numbers. For the 

 possible combinations of two sets of gametes, each G and W, are represented by their 

 product, GG+2GW + WW, or, simplified as already explained, G+2 G(W)+W. 



The principle illustrated by these examples is, as pointed out by Bateson (:Q2), 

 the most fundamental and far-reaching of the Mendelian ideas. It is known as the 

 "law of segregation," or " splitting " (de Vries, :00) of the parental characters at gamete- 

 formation, or as the "principle of gametic purity" (Bateson, :02). Dominance is 

 purely a secondary matter; it may or may not occur along with segregation, though 

 the latter can be more easily demonstrated in cases where it is associated with the 

 former. The principle of gametic purity rests upon the assumption that gamete- 

 formation is the reverse of fertilization. In fertilization gametes A and B unite to 

 form a zygote AB; when this zygote in turn forms gametes, they will be again A and 

 B. From a knowledge of the somatic form alone of pure A's and B's, one can make 

 no trustworthy prediction as to the form of AB. (Here is the fundamental error 

 of the "law of ancestral heredity" as stated by Galton ('97) and by Pearson (:03). 

 AB may have invariably the somatic form of A or of B (cases of simple dominance, 

 as of gray over white in mice); or it may have sometimes the form of A, sometimes 

 that of B (cases of alternative dominance — see Tschermak, :00); or, finally, the 

 somatic form of AB may be different from both that of A and that of B (cases like 

 that of the gray hybrid formed by the cross of black-white with white mice). But, 

 no matter what the somatic form of AB is, we may with confidence predict that its 

 gametes will be essentially pure A's and J5's, and in equal proportions. This is the 

 Mendelian expectation in all cases of alternative inheritance. Whether it applies 

 to other cases also, and, if so, to what extent, is not yet known. 



III. COMPLETE ALBINISM A RECESSIVE CHARACTER. 



i. In Mice The recessive nature of complete albinism in mice has been con- 

 clusively demonstrated by Cuenot (:02), who, on crossing wild gray house-mice with 

 albinos, obtained always gray mice indistinguishable in appearance from the pig- 

 mented parent. Yet these gray hybrids, when bred inter se, produced both gray 



