34 HEREDITY IN RABBITS, RATS, AND MICE. 



over gamete. This makes a total of 71 such gametes. Cross-over 

 gametes were represented singly in each of the 17 individuals which 

 were heterozygous for pink and doubly in the one which was homo- 

 zygous for pink. This makes a total of 19 cross-over as against 71 

 non-cross-over gametes, which is 21.1 per cent cross-overs. This is an 

 indicated linkage strength a little less close than that between pink- 

 eyed and red-eyed yellow, in which case the percentage of cross-over 

 gametes was estimated at 18.3. For with no-linkage gi'v'ing 50 per 

 cent cross-overs, it is evident that the linkage strength increases as 

 the percentage of cross-overs decreases until (when cross-overs cease) 

 linkage becomes complete. If we measure the strength of linkage by 

 the difference between the observed percentage of cross-overs and 

 50 per cent (the percentage of cross-overs when no linkage occurs), 

 then linkage between red-eyed j^ellow and pink-eyed yellow is 31.7 and 

 that between pink-eyed yellow and albinism is 28.9, as provisionally 

 determined. 



The linkage between red-eyed yellow and albinism is much stronger 

 than the linkage in either of the cases just discussed. Tests have 

 been made for the presence of the red-eyed yellow gene in 160 F2 

 albinos and for the presence of albinism in 57 Fo red-eyed yellows 

 derived from the cross of albino with red-eyed yellow. Only a single 

 cross-over has been detected, and even that is not beyond question. 

 One of the F2 albinos, a male, when mated with a pure red-eyed yellow 

 female, sired a litter of young, all of which were dark-eyed except one. 

 This one proved to be a yellow but died, as did the father, before 

 additional breeding tests could be applied. If this yellow individual 

 was really sired by the albino male (and not accidentally introduced 

 from some other cage, a remote possibility), then that male evidently 

 carried yellow as well as albinism and in his genesis a cross-over 

 gamete must have functioned. Each of the other F2 albinos and the 

 F2 yellows tested manifestly arose from the union of gametes neither 

 of which transmitted both yellow and albinism, since as mated they 

 produced only dark-eyed young (4 or more each). On these assump- 

 tions the experiments thus far show that only one gamete out of 434 

 formed by Fi parents can have been a cross-over gamete, which 

 apparently gives less than one per cent of cross-overs. 



The experiments are being continued with the hope of finding addi- 

 tional cross-overs and of thus securing a double recessive race, which 

 will make possible a more accurate determination of the linkage 

 strength. The information already presented shows that on the 

 chromosome theory the genes for albinism and for red-eyed yellow are 

 extremely close to each other in the same chromosome and that the 

 gene for pink-eyed yellow, while lying in this same chromosome, is 

 at some distance from the genes for albinism and red-eyed yellow. 



