180 



INHERITANCE IN RATS. 



gametes, would give an expectation of 29.9 black-eyed : 13.6 red-eyed : 

 14.5 albinos, which agrees much better with the observed numbers 

 (30 : 18 : 10) than does the 9:3:4 distribution. But if red-eye is 

 linked with albinism as well as with pink-eye, then albinism and pink- 

 eye should be linked with each other. Apparently such is the case, 

 for three F 2 litters from the cross pink-eye X albino include 12 black, 

 12 pink-eyed, and 3 albino young. A 9:3:4 ratio (expected if no 

 linkage occurs) would call for 15 black, 5 pink-eyed, and 7 albinos. 

 Linkage of 5 : 1 would call for 14 : 6 : 7, and perfect linkage would call 

 for 14 : 7 : 7. It is evident that the observed numbers of blacks and 

 albinos are too small on any of these hypotheses, but the existence of 

 linkage would tend to diminish the number of blacks and albinos in 

 proportion to the number of pink-eyed, which is the nature of the 

 deviation observed. To determine definitely whether linkage really 

 occurs between the yellow variations and albinism, and if so, what is 

 its strength, further experiments are needed, which are now in progress. 

 It will also be desirable to determine whether the linkage strength is 

 the same in both sexes. 



SUMMARY. 



Two yellow variations in rats which have recently arisen as muta- 

 tions show mutual repulsion in heredity. When crossed with each 

 other they produce an F! generation composed exclusively of rever- 

 sionary dark-eyed individuals. The F 2 young are of three apparent 

 classes, dark-eyed, red-eyed, and pink-eyed. Their numerical propor- 

 tions deviate somewhat from the typical 9:3:4 ratio. Further, the 

 proportions of the several expected classes of red-eyed and pink-eyed 

 young do not agree with those usually observed in an F 2 Mendelian 

 population. But in both cases the deviations are largely accounted for 

 by the supposition that the genes of the respective yellow variations 

 are "linked" (in this case showing repulsion) and that the proportion 

 of "cross-over " gametes is about 17 per cent, or in other words, that non- 

 cross-over gametes are about 4.8 times as numerous as cross-over gametes. 



NOTE. In the foregoing discussion it has been assumed that the ratio of 

 cross-over to non-cross-over gametes is the same among gametes which take 

 part in producing yellows as among those which take part in producing black- 

 eyed individuals. Theoretically it should be slightly different, as the following 

 table will show: 



