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THE AMERICAN NATURALIST [Vol. XLIX 



Fj, and black-and-tans and blacks in F 2 . This result 

 shows that black-and-tan is a simple dominant over black. 



To establish the allelomorphism of black-and-tan with 

 gray the following experiments may be cited. A black- 

 and-tan rabbit heterozygous for black was crossed with 

 a pure-bred Belgian hare, which variety possesses the 

 genetic color factors of wild rabbits, including the ordi- 

 nary agouti factor. All the F x young were gray, closely 

 resembling Belgian hares, but proved to be genetically 

 of two types. For, when mated with black rabbits, some 

 of them produced gray young and black young, while 

 others (even when mated, as in some cases, with the same 

 black animals) produced gray young and black-and-tan 

 young. This result was quite what was to be expected if 

 gray, black-and-tan and black are mutually allelomorphic 

 conditions. On no other hypothesis which we can sug- 

 gest was it to be expected. For the black-and-tan parent 

 in the cross was known to be heterozygous for black. It 

 accordingly should form two sorts of gametes, black and 

 black-and-tan respectively, provided that these conditions 

 are allelomorphic to each other. The Belgian hare parent 

 was known to transmit gray in all its gametes. The com- 

 binations expected from the cross are therefore of two 

 types, viz.: (1) gray combined with black, and (2) gray 

 combined with black-and-tan. It is well known that gray 

 and black are allelomorphs of each other, the former being 

 dominant. Zygotes of type (1), therefore, should pro- 

 duce gametes of two sorts, gray and black; and when 

 back-crossed with black should produce equal numbers of 

 gray young and black ones but no black-and-tan young. 

 We have tested 12 F 1 gray young from this cross (6 males 

 and 6 females) which are evidently of type (1). Mated 

 with black animals, they have produced 69 gray young, 

 and 65 black ones, but no black-and-tans. 



- On the other hand 8 F x gray rabbits from the cross 

 under discussion have proved to be of type (2), producing 

 gray young and black-and-tan young but no black ones. 

 Together they have produced 44 gray and 51 black-and- 

 tan young, besides 14 other young (two litters) which 



