GAMETIC COUPLING IN YELLOW RATS. 



Two yellow-coated varieties of the Norway rat (Mus norvegicus) 

 made their appearance as sports or mutations in England a few years 

 since (Castle, 1914) and are now recognized as distinct varieties by 

 fanciers. Both are similar in appearance except for the eye color. In 

 one variety the eye is pink, showing under gross inspection only the 

 color of the blood in the retina. In the other variety the eye is a 

 reddish-black, owing to the combined effect of the red-colored blood 

 and the black-pigmented retina. Since the retinal pigment is much less 

 in this variety than in rats with gray or black coats, the eye is redder. 

 It will be convenient to distinguish the dark-eyed yellow variety as 

 red-eyed, reserving the name black-eyed for gray or black rats. 



In the coats of both the pink-eyed and the red-eyed varieties of 

 yellow rats black pigment is very feebly developed. It is in fact of a 

 pale cream color. But the true yellow pigment seen on the tips of the 

 hairs of gray rats is retained in full intensity in the yellow varieties. 

 For this reason agouti varieties of yellow rats are much brighter-colored 

 than non-agouti varieties. A non-agouti yellow variety has fur cream- 

 colored throughout its length ; the corresponding agouti variety has fur 

 of this same cream color at its base, where the fur of gray rats is black- 

 pigmented, but the hair-tips are of a bright yellow color of exactly the 

 same shade as the hair-tips of gray rats. Hence it is clear that in 

 these yellow varieties of rats a genetic factor for black pigmentation has 

 been affected without any apparent change in the genetic apparatus for 

 producing ordinary yellow pigment. 



This is quite different from the genesis of yellow coat in most ro- 

 dents for example, in guinea-pigs and rabbits in which black pig- 

 ment is not apparently changed in character but merely in distribution, 

 being "restricted" chiefly to the eye. In the yellow varieties of rats 

 black pigment seems to be affected in the same way as in the pink-eyed 

 variety of guinea-pigs and mice, viz, to be greatly weakened without 

 affecting in the least the development of yellow pigment. The genetic 

 behavior as well as the appearance of the pink-eyed yellow variation in 

 rats is in every way parallel with the behavior of the variations known 

 by the same name in mice and guinea-pigs. But red-eyed yellow in 

 rats is a genetically distinct variation, as we shall presently see. In no 

 other mammal does there occur a parallel variation, so far as I know. 

 Both red-eyed yellow and pink-eyed yellow were found to be recessive 

 Mendelian variations in crosses with black-eyed rats. From a cross 

 between black-eyed and red-eyed an F 2 generation of ^609 rats was 

 raised, of which 452 were black-eyed and 157 red-eyed; expected, 

 457 : 152. From a cross between black-eyed and pink-eyed rats, cer- 

 tain F! females were back-crossed with a pure pink-eyed male. They 

 produced 46 black-eyed and 39 pink-eyed ; expected, 42 of each. 



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