CROSSES WITH JAPANESE WALTZING MICE. 



43 



of a tortoise animal we should have an animal yellow-and-white in appearance, 

 though gametically a pure tri-color (tortoise and white). This condition is 

 found in guinea-pigs where two yellow (red) and white animals when crossed 

 together have given young with black on the coat (Castle, 1912), and we know 

 that one of these animals at least must have the gametic composition of a tri- 

 color. In a case of this sort the yellow-and-white animals would occur in 

 ratios entirely free from any mendelian principle, and would give a spurious 

 reversal of dominance. 



Perhaps one of the two foregoing suggestions may serve to throw some light 

 on the results upon which Galton's "law" of heredity was based. It must be 

 remembered that tortoise-shell animals are in their gametic composition no more 

 intermediates between black (extended) and yellow (restricted) animals than 

 forms spotted with white are intermediate between colored forms and albinos. 



CROSSES WITH JAPANESE WALTZING MICE. 



Morgan (1911a) has mentioned the fact that in crosses between self and 

 spotted forms, in mice, the spotting seemed modified or contaminated by the 

 cross. He gives no figures, however, in support of this statement and fails 

 to mention that the modification of spotted forms by crossing with self was 

 reported by MacCurdy and Castle (1907). 



Dr. Castle and the writer have examined carefully the degree of pigmen- 

 tation of a race of pure Japanese spotted mice and have estimated the percent- 

 age of colored area on the dorsal surface ; 72 animals were thus examined. The 

 average pigmentation was 32 per cent. The range of pigmentation was from 

 20 to 55 per cent. Animals of this pure Japanese race were crossed with a 

 uniform self-pigmented race of homozygous dilute brown animals, known to 

 produce no spotted or albino young. Fi from these animals consisted of all self 

 pigmented young showing no spots of white. Certain of these Fi animals were 

 crossed inter se to obtain F2, others were crossed back with females of the pure 

 Japanese race. 



In the first case F2 was found to consist of 45 self animals (no spots) and 21 

 animals having some white hairs or spots on the dorsal surface. The 3 to 1 

 ratio is here approximated, the exact theoretical expectation being 49.5 self 

 to 16.5 spotted. The extracted spotted animals differ widely from the original 

 Japanese race from which they inherited the spotted-coat character. A com- 

 parison of the two categories of spotted forms (together with the back cross, 

 which will be treated later) will be seen in the following table : 



