416 THE AMERICAN NATURALIST [Vol. LYI 



From these considerations we may regard it as not 

 unlikely that my "pallid" race of Peromyscus has re- 

 sulted from the mutation of a genetic factor homologous 

 with that which has mutated in the case of Castle's "red- 

 eyed yellow" rats. 



This decisive result, as regards the existence of link- 

 age between the pallid and albino factors in Peromyscus, 

 stands in contrast with the apparent absence of such 

 linkage in another cross between mutant strains of these 

 mice. Albinos were mated with mice belonging to a 

 strain which I have elsewhere referred to rather inap- 

 propriately as "yellows." ^2 ^he latter vary from clay 

 color to a distinctly reddish hue, according to the strain, 

 and are characterized primarily by a marked increase in 

 the length of the "agouti" cross-band and by a decrease 

 in the proportionate number of all-black (unhanded) 

 hairs in the pelage. Where present, however, the black 

 pigment is of full intensity. This applies to the basal 

 zone of the body hairs, both dorsal and ventral, to the 

 black hairg of the dorsal tail stripe, as well as to the 

 eyes, ears and soles of the feet. 



Matings between albinos and "yellows" have resulted 

 exclusively in Fj mice of the wild type (dark). An Fg 

 generation of 83 was obtained, consisting of 52 dark 

 individuals, 13 yellows and 18 albinos. On the assump- 

 tion of purely random assortment of gametes, the "ex- 

 pected" numbers are 44, 15 and 20, respectively. The 

 observed numbers are doubtless within the range of ' ' ac- 

 cidental" variability. In any case they give no evidence 



repeated, howeTsr, that we are not here dealing \vith cases in which there 



the same individual. The odds in favor of this (linkage aside) may, as 

 stated above, be as high as 2 to 1, or even 3 to 1. Thus, the likelihood of 

 obtaining, by chance alone, 17 non-cross over cases out of 18 becomefl 

 vanishingly small. 



12 Genetics, May, 1917; American Naturalist, August-September, 1918. 

 Collins and myself. ' g p pe by ^ . 



