28 HEREDITY IN RABBITS, RATS, AND MICE. 



hence there is no evidence that cross-over gametes are produced as 

 often as once in 192 times. It, therefore appears that English and 

 Dutch are either very closely coupled or are variants of the same 

 locus. As regards the effect of the English cross on the grade of the 

 Dutch character, this is indicated in the grade of the back-cross Dutch 

 young, which range in grade from 6 to 17, average 13.88. Similarly 

 produced back-cross Dutch obtained from FI (white X self) X white 

 (table 17) range from 13 to 17 and are of mean grade 15.15. This 

 indicates that the English cross has darkened white Dutch even more 

 than self did in a similar cross (table 27). The superior darkening 

 effects of English over the self used in table 27 may be attributed to 

 the more highly selected character (for darkness) of the English used 

 in the cross and to the less highly selected character (for whiteness) 

 of the Dutch used in the same cross, but the difference is not great. 



At any rate, from the grade distribution of the young produced in 

 the back-cross of FI (English X white Dutch) X white Dutch, it is 

 clear (1) that English and Dutch behave like allelomorphs, or closely 

 linked factors, since in nearly 200 cases studied no cross-over is 

 observed, i.e., no gamete transmitting neither English nor Dutch; 

 (2) that the segregated English and Dutch borne by the gametes of 

 FI individuals are mutually modified, the English (previously selected 

 for darkness) being made lighter, and the Dutch (previously selected 

 for whiteness) being made darker. In these mutual modifications we 

 are dealing probably for the most part with residual heredity, but 

 it is possible that quantitative variation of the English and the Dutch 

 genes is in part responsible, yet such an interpretation is not favored 

 by the results obtained from the critical experiments with hooded rats, 

 which point strongly to changed residual heredity as the correct 

 explanation of changed phenotypes, when only a single Mendelizing 

 character can be observed. 



