52 GENETIC STUDIES OF RABBITS AND RATS. 



not one of the young was mottled like the sire. This occasioned no 

 surprise when the mates were not related to the tri-color male or 

 were not his direct descendants, for it was conjectured that he might 

 be a mutant due to a new recessive gene, which accordingly would 

 only become visible when in a homozygous state. But if he him- 

 self represented such a homozygous recessive combination, all his 

 gametes should transmit the mottled condition, and all his daugh- 

 ters consequently should be heterozygous for the mottled condition, 

 and in matings with their sire should produce 50 per cent of mottled 

 individuals. The fact that they do not produce mottled individuals 

 shows this hypothesis to be untenable. 



The tri-color male, in fact, breeds like his gray-hooded brothers 

 and sisters, producing gametes which are commonly either c or p, 

 but which in no case transmit a mosaic relationship. From his 

 ancestry and from the results of his matings, we know him to be a 

 double heterozygote, CcPp. The Mendelian expectation is that he 

 will form four kinds of gametes, cP, Cp, CP, and cp, but since there 

 exists linkage (in this case repulsion) between c and p, the first two 

 classes of gametes will be commoner than the other two. His 

 breeding behavior accords with these expectations. In most of his 

 gametes he transmits either albinism or pink-eyed yellow. In an 

 occasional gamete he probably transmits both, a matter which has 

 not been tested, as it would require special matings. It is certain 

 that in an occasional gamete he transmits neither c nor p, a condition 

 expressed in the formula CP, which like cp would represent a cross- 

 over. His gametes, accordingly, appear to be such as are normally 

 produced by FI doubly heterozygous individuals like his gray and 

 gray-hooded brothers and sisters. 



How, then, can we account for the production of yellow areas in 

 the gray coat? By an explanation similar to that which Morgan 

 and Bridges (1919 1 ) have given to account for the production of 

 gynandromorphs in Drosophila. We may suppose that at an early 

 cleavage (perhaps the first) of the fertilized egg from which this rat 

 developed, one of the pair of chromosomes, in which the genes c 

 and p are borne, failed to divide as normally, or that for some other 

 reason it failed to pass into one of the cell-products. Further con- 

 sideration shows that it must have been the maternal rather than 

 the paternal chromosome of this pair which was lost. For the mother 

 was an albino and must have contributed cP, but the father was a 

 pink-eyed yellow, Cp, and his contribution by itself would produce 

 yellow fur, whereas with the mother's contribution it would pro- 

 duce gray. We may suppose, accordingly, that a blastomere con- 

 taining only the paternal contribution produced that part of the 



Carnegie Inst. Wash. Pub. No. 278. 



