COLOR INHERITANCE IN MAMMALS 



III. 



The Rat — Few Variations of Factors Known until Recently — Castle's Selection 

 Experiment Any Interpretation of It Demonstrates the Efficacy of 

 Darwinian Selection 



Skwall Wright 

 Bureau of Animal Industry, Washington, D. C. 



UNTIL recently rats were rela- 

 tively poor in known color 

 varieties. Blacks, albinos and 

 hooded blacks and grays were 

 the only varieties known besides the 

 wild Kra>' color. Three recessive Men- 

 delian factors, called here Sh, Ca and a, 

 were demonstrated early by the work of 

 Bateson,^ Doncastcr,- MacCurdy and 

 Castle,^ and others. These obviously 

 are variations of a similar nature to 

 piebald, albinism and black (or rather 

 loss of agouti ])attern) in mice and are 

 to be assigned to classes lao, lb and 2ai, 

 respectively. 



In 19 14, Castle^ reported on two new 

 variations which had a])pearcd inde- 

 pendently as sports among wild rats in 

 England. They are of rather similar 

 apperance, the gray color of the wild 

 rat in both cases being changed to a 

 yellow, in which, however, the base of 

 the hair is a pale slaty color. When 

 the agouti factor is replaced by a, 

 the entire fur is of this pale slaty color. 

 The only distinct difference between the 

 two variations is that in one the eyes 

 are reduced to pink, in the other merely 

 to a dark red. Both of them behave as 

 simple recessives. In spite of their 

 great similarity in effect Castle found 

 them to be independent variations, 

 reproducing a gray only a little less 

 intense than the wild gray when crossed 

 together. In fact, it was found difficult 

 to get them into the same gamete and 

 Castle and Wright'' reported on a very 

 high degree of linkage l)etween them. 

 Castle" further found indications that 



both of them are linked with albinism. 

 This would be of very great interest if 

 confirmed by further work in view of 

 the ])roljable linkage between albinism 

 and the pink-eyed factor j) of mice 

 in the data of Darbishire and Cuenot. 

 The pink-eye factor of mice is the 

 nearest homologue of the pink-eye 

 factor of rats. Both reduce black to a 

 pale slaty brown in the fur, and nearly 

 eliminate it in the eye but do not affect 

 yellow. In agoutis there seems also 

 to be some widening of the agouti band , 

 due to the weakening of the black. 



Thus the appearance of the red-eyed 

 and pink-eyed agoutis is practically 

 yellow. Both factors, however, must 

 be placed in class 2b as being more 

 sitnilar in their action to the brown and 

 nink-eved colored variations of other 



» Bateson, W. 100.^. Prnc. Zool. Soc., 2:71-78. 



' Doncaster, L. \WF>. Froc. Camh. Phil. Soc, 13:215-227. 



» MacCurdv, H., and Castle, W. E. 1907. Cant. Inst. HV/.s7;. Piih. No. 70, 50 ]>p 



* Castle, W. E. 1914. Amer. Nat., 48:65-73. 



'Castle, W. E., and S. Wright. 1915. Sci. N. S., 42:193-195. 



•Castle, W. E. 1916. Cam. Inst. Wash. Pub. No. 241, part iii:161-KS7. 



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