OCCURRENCE OF LINKAGE IN RATS AND MICE. 33 



SO we are forced to conclude that individual differences as regards the 

 production of many or few cross-overs have not been shown to exist. 



In publication 241 evidence was presented indicatmg that albinism 

 in rats is probably due to a gene which is linked with the genes for red- 

 eyed yellow and pink-eyed yellow. This idea is now fully established 

 and we are able to give provisional estimates of the linkage strengths 

 involved, although the in\'estigation of this matter is still incomplete. 



Wlien red-eyed and pink-eyed rats are crossed with each other, or 

 either sort is crossed with an albino, the Fi young produced are dark- 

 eyed and dark-coated (either black or gray, according as the agouti 

 factor is absent or present). But the Fi young are not quite as dark 

 in color as wild rats. This shows that all three variations are reces- 

 sive and complementary, but that the allelomorph of each is a little 

 less effective m producing pigment when in heterozj-gous form than 

 when in homozygous form (as in wild rats, or in Irish or in hooded 

 rats). Fi individuals from the cross of albino with pink-ej^ed yellow, 

 when bred with each other, produce an F2 generation of three apparent 

 types, viz, (1) dark, in eye and coat color; (2) pink-eyed yellow; 

 (3) albino. If no linkage occurred we should expect these three 

 classes to occur in the ratio 9:3:4; but, as was pointed out in 

 pubHcation 241, linkage would tend to equalize the numbers of pink- 

 eyed and albino young, and such a tendency has been recorded. 

 Further, if no linkage occurs, but if pink-ej^ed yellow and albinism 

 segregate quite independently of each other, then half the albino 

 gametes formed by Fi individuals should transmit pink-eyed yellow 

 and half should not transmit it; conversely, half the gametes which 

 transmit pink-eyed yellow should also transmit albinism and half 

 should not. If less than half the gametes which transmit one character 

 transmit the other, the two show repulsion. 



To test the matter, 45 F2 albinos have been mated with homozygous 

 pink-eyed individuals. Of the 45 so mated, 17 have produced both 

 pink-eyed and dark-eyed young, one has produced only pink-eyed 

 young, and 27 have produced only dark-eyed young. The 17 are 

 e^'idently heterozygous for pink-eye as well as homozygous for albinism, 

 their formula being ccpP. The one which produced only pink-eyed 

 young is probably of the formula ccpp. The 27 which produced only 

 dark-eyed young are of the formula ccPP. 



We may now consider what was the nature of the gametes which 

 produced these 45 individuals. A gamete which furnished both 

 albinism and pink may be called a cross-over gamete; one which 

 furnished albinism only must be regarded as a non-cross-over gamete. 

 The 27 albinos which did not transmit pmk-eye evidently arose each 

 from the union of two non-cross-over gametes. This accounts for 

 2 X 27 or 54 non-cross-over gametes. The 17 individuals which 

 were heterozygous for pink evidently received each a single non-cross- 



