Species crosses in Rats. 1 09 



brothers or sisters having: only E or F, and heterozygous for that one 

 g:ene. A whitetipped bhick male mated to a black sister gave 3 young 

 with white tips and 3 without. A whitetipped agouti female to a 

 normal black i>rother gave 4 whitetipped to 3 with solidly pig- 

 mented tails. 



In the inheritance of yellow we meet the same thing. Almost in 

 all crosses the jellows show to be different in one gene only from 

 blacks and agoutis. This must mean, that these cd animals were seldom 

 mated to CD animals but almost always with animals having only C or 

 only D. From the origin of the yellow's we know that probably some 

 species have C, and are fully coloured for that reason, whereas other 

 wild species have D and are agouti instead of yellow accordingly. 

 A third possibility is the simultaneous presence of C and D in one 

 species. 



Backcrosses to wild species. 



Yellows were mated both to houserats and treerats in Java. From 

 the cross between a whitebellied yellow^ male to a houserat female we 

 obtained 8 young, all whitebellied agouti, of which only three males 

 survived. Mated back to yellow, one of these gave in two litters 

 3 whitel)ellied yellow, 2 darkbellied yellow, 2 darkbellied ag'outi and 



2 whitebellied agouti. Obviously, the houserat contains only one of the 

 two genes, C or I), and the cross was either cd > Cd or cd X cD. 

 The houserat has a dark belly. 



One litter of 9 was obtained from a treerat and a yellow female. 

 All were whiteln'Uied agouti. One pair of the Fi"s was mated inter se, 

 producing 17 whitebellied agoutis and 1 yellow whitebellied. 



The young were born in a tank and the number of litters was 

 not noted. In this case the proportion of yellows in F2, which was one 

 among eighteen, proved the treerat to possess both factors C and D. 



Chocolate. 



We never had more than one adult chocolate animal, a female, 

 which refused to breed in Holland, but which in Java produced two 

 litters. The first litter was from a whitebellied agouti male, 7 black 

 young. This shows the presence in chocolate of factor A. These young 

 ••died at a low age. The second litter was from a whitebellied yellow 

 male, 9 blacks, of w^hich one pair was raised. This pair had one litter, 



3 blacks and 1 darkbellied yellow, of a very deep shade. From analogy 



