No. 646] LINKAGE IN PEROMYSCUS 413 



belonging to widely dilferent families.^ In one case, 

 that of the mutation known as pink-eye," not only is 

 the visible modification closely similar in rats and mice, 

 but the linkage relations between this factor and that 

 for albinism are known to be of the same order of mag- 

 nitude in the two animals.'* 



Some years ago, Castle^ described two similar muta- 

 tions in the Norway rat, which he termed ''pink-eyed 

 yellow" and "red-eyed yellow," respectively. These, 

 according to the published descriptions, differ chiefly in 

 the color of the eyes, the latter variety having darker 

 eyes than the former. These two mutations, and like- 

 wise true albinism, were all found to result from the 

 modification of distinct genetic factors. Any two of 

 them, when crossed, gave rise to the wild type in the 

 first hybrid generation. On the other hand, further 

 breeding tests led Castle to conclude that all three of 

 these factors were linked. When red-eyed and pink-eyed 

 rats were interbred, the cross-over percentage proved to 

 be about 18. When pink-eyed rats were crossed wath 

 albinos, this value proved to be about 21. On the other 

 hand, the linkage between red-eye and albino proved to 

 be almost absolute. -One hundred and sixty Fg albinos 

 and 57 red-eyed yellows, when mated with pure red- 

 eyes and albinos, respectively, yielded but a single off- 

 spring which was not of the wild type. 



More recently Dunn^ has tested the linkage between 

 this same red-eyed condition and albinism in the rat. 

 From his o^\ti data he computes a cross-over value of 

 1.8 per cent., but when his data are combined with those 

 of Castle, this value falls to less than one per cent. 



Castle and Dunn have likewise tested the degree of 

 linkage between "pink-eye" and albinism in the mouse 



