PART II. 



ON A NON-TRANSMISSIBLE TRI-COLOR VARIATION 



IN RATS. 



In the course of studies of linkage in rats, I have observed the 

 production of a tri-color individual, gray, yellow, and white, similar 

 to the tri-color varieties of guinea-pig, and I had hoped to establish 

 from it a new color variety of rat, but thus far no success has at- 

 tended my efforts. The case, in its bearings on the nature and origin 

 of variations, is not without interest, and so will be described briefly. 



The two genes whose linkage relations were being investigated 

 when the tri-color individual made its appearance have been desig- 

 nated c and p. Both are recessive in crosses and thus become visible 

 as somatic characters only when present in the homozygous state, 

 cc or pp respectively. An individual of the formula cc is an albino; 

 an individual of the formula pp is pink-eyed and yellow-coated. 



When an ordinary albino is crossed with a pink-eyed yellow indi- 

 vidual, young are produced which are neither albinos nor pink-eyed 

 yellow, since neither c nor p will be homozygous in the cross-bred 

 individuals, which are in fact gray in color like wild rats, or else 

 gray-hooded, if the gene for hooded pattern is present in homozygous 

 condition. By means of such crosses between albino and pink-eyed 

 yellow rats, gray and gray-hooded young were being produced when 

 the tri-color individual made its appearance. It is a gray-hooded 

 individual, like its brothers and sisters, except that the areas nor- 

 mally gray are liberally mottled with yellow. The mottling extends 

 practically throughout the colored portions of the coat from nose 

 to tail tip. The yellow areas vary in size from those which contain 

 merely a few yellow hairs to those of a square inch in extent. The 

 least yellow is found on the right shoulder, which is almost like 

 normal gray in appearance. The most yellow is found in the middle 

 of the back, to the left of the median line, where a large spot of 

 clear yellow occurs in the wide back-stripe of the hooded pattern, 

 which would correspond roughly with grade +2^ of the grading 

 scale used in our studies of hooded rats. 



The tri-color individual is, fortunately for the purposes of genetic 

 study, a male. He has been mated with albinos, with pink-eyed 

 yellows, and with gray FI cross-breds between the albino and the 

 pink-eyed yellow races, and later with his own daughters of the 

 colors white, pink-eyed yellow, and gray. Hundreds of young have 

 been produced by these matings, for the animal is a remarkably 

 large and vigorous one and his mates have proved very prolific, but 



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