DUNN 



at about the same relative distance 

 apart. This localizes both of these 

 genes in both species, and leads to 

 some interesting conclusions and spec- 

 ulations which will be more fully con- 

 sidered later. In guinea-pigs there is 

 incomplete evidence concerning the 

 location of the gene for pink-eye but 

 some data which Dr. Sewall Wright 

 has kindly extracted from his breeding 

 records and sent to me indicate that 

 the locus of pink-eye is not in the 

 albino chromosome but elsewhere. As 

 we shall see, this may prove just as 

 instructive concerning the homologies 

 between species in germinal constitu- 

 tion as the more definite localization 

 of the gene in rats and mice. 



YELLOW 



The self or solid yellow coat colora- 

 tion in rodents appears to be divisible 

 as to its cause into two different cate- 

 gories. In the first of these may be 

 placed those yellow varieties which 

 have arisen by a change in a gene 

 governing the extension of black and 

 brown to the fur, and the alternative 

 (allelomorphic) condition of restric- 

 tion of these melanic pigments to the 

 eye, while the pelt is yellow. In the 

 presence of this gene (restriction [r]) 

 the melanic pigments are probably not 

 produced in sufficient amount or to a 

 sufficient intensity to invade the fur, 

 leaving the residual yellow which is 

 present in all "agouti" animals in pos- 

 session of the whole extent of the hair. 

 This gene is recessive to full extension 

 and is distinct in its inheritance from 

 the gene which determines the barring 

 of each hair in the agouti pattern. Ani- 

 mals may possess the gene for yellow, 

 with the gene for "agouti" or with- 

 out it. "Agouti" yellows have much 

 lighter bellies than non-agouti yellows. 

 Restricted yellow occurs in the fol- 

 lowing rodents: 



99 



Leporid<e — Oryctolagus cuniculus 

 Murid^ — *Microtus pennsylvanicus 

 Peromyscus maniculatus 



gambeli 

 Rattus rattus 

 *Rattus alexandrinus X R. 



Caviidee — 



rattus 

 Cavia cobaya 



The distinction between this yellow 

 and the second type, about to be de- 

 scribed, is made on grounds of the 

 mode of inheritance only, for the ap- 

 pearance of the latter type is identical 

 with that of the former. The pelt of 

 the second type is yellow and the eyes 

 are dark. The gene which differen- 

 tiates it is however dominant over its 

 allelomorphs agouti and non-agouti. 

 This dominant yellow is known only 

 in the house mouse and because of its 

 peculiar mode of inheritance has been 

 the subject of considerable research 

 through a combination of genetic and 

 embryological methods to which 

 Castle, Little, Kirkham and others have 

 contributed. Such investigations have 

 established the following facts: 



( 1 ) Yellow house mice do not 

 breed true but when bred together 

 always produce yellow and non-yel- 

 low young in the ratio of 2:1. 



(2) Litters from yellow by yellow 

 are on the average 25 per cent smaller 

 than litters from non-yellow varieties. 



(3) In the uteri of yellow females 

 pregnant by yellow males there have 

 been found disintegrating embryos ap- 

 proaching 25 per cent of the total 

 embryos. 



Ordinary heterozygotes (hybrids in 

 one character) when bred together 

 produce 25 per cent pure dominants, 

 50 per cent heterozygotes, and 25 per 

 cent pure recessives. In the offspring 

 of yellow mice the two latter classes 

 have appeared in the expected propor- 

 tions; pure dominant yellows have 

 never been disclosed by breeding tests. 



