OCCURRENCE OF LINKAGE IN RATS AND MICE. 35 



In mice it has been shown by Haldane et al. that the genes for albin- 

 ism and for pink-eye are probably linked with each other. This fact, 

 interesting in itself, is made doubly so by the consideration that 

 characters apparently identical in nature with these two are also 

 linked with each other in rats. Since mouse and rat are species grouped 

 by systematists in a single genus, it should be of interest to compare 

 their genetic constitution as fully as possible. With this idea in mind 

 we had already undertaken to study the linkage relations of albinism 

 and pink-eye in mice before the appearance of the paper by Haldane 

 et al. 1 This investigation was undertaken by Mr. L. C. Dunn while 

 acting as my assistant. Upon his entering military service, I took 

 over the experiments. It is a pleasure to acknowledge Mr. Dunn's 

 important part in the work. 



We began, as in the rat experiments, by crossing pink-eyed with 

 albino individuals. Dark-eyed FI young were produced exactly as 

 in rats. These bred with each other produced an F 2 generation of 

 dark-eyed young, pink-eyed young, and albinos, in a 9 : 3 : 4 ratio 

 manifestly modified by linkage. Pink-eyed F 2 individuals were tested 

 for the presence of albinism and albino F 2 individuals were tested for 

 the presence of the pink-eye gene as a first step toward the production 

 of a race of double recessives needed to ascertain the proportion of 

 cross-over gametes formed by FI individuals. The simplest way of 

 making the tests was found to be the mating of F 2 albinos with F 2 

 pink-eyed individuals. This afforded simultaneously a test of both 

 parents. For if the pink-eyed parent carried albinism, 50 per cent of 

 the young would be albinos, otherwise none would be albinos. But 

 if the albino carried the pink-eye gene, 50 per cent of the young pro- 

 duced would be pink-eyed. If both these contingencies were realized 

 in the mating, 25 per cent of the young would be pink-eyed and 25 per 

 cent albinos. All other young, as in a cross of pure pink-eyed with 

 pure albinos, would be dark-eyed. 



If no linkage occurred between pink-eye and albinism, it would be 

 expected that half the F 2 pink-eyed individuals would carry albinism, 

 and also that half the albinos would carry pink-eye. Any smaller 

 proportions than these of pink-eyed carrying albinism or of albinos 

 carrying pink-eye, among the F 2 individuals, would indicate linkage. 



Linkage is very clearly shown by the tests made. Among 63 F 2 

 pink-eyed which were tested, 18 produced each one or more albino 

 young in litters otherwise dark-eyed, while 45 produced no albinos 

 but only dark-eyed young. In the genesis of the 18 parents men- 

 tioned, it is evident that 18 cross-over gametes had united with 18 



1 In fact, I have not yet had access to the paper by Haldane et al. but know it only as cited 

 by others. Our copy of the journal in which it appeared is probably at the bottom of the ocean 

 and we have been unable as yet to replace it. 



