58 HELEN DEAN KING 



found to be a dominant, as is the case in various ulotrichous 

 races of man, in European races of white stock (Mohr, '32; 

 Schokking, '34), in karakul sheep (Macalik, '32; Tanzer, '32), 

 in swine (Rhoad, '34), in caracul mice (Carnochan, '37; Dunn, 

 '37), as well as in curly and curly 2 rats (King, '32; Gregory 

 and Blunn, '36). On the other hand, if the wave in the hair 

 is slight or tends to disappear with advancing age, the muta- 

 tion, seemingly, is always a recessive, as in the rex rabbit 

 (Castle, '29), kinky rats (Feldman, '35), and wavy mice 

 (Crew, '33; Keeler,* '35). 



After the appearance of the curly mutation it was decided 

 that an accumulation of mutant genes in the stock was un- 

 desirable, since such genes might possibly affect growth or 

 other life processes in individuals carrying them. Therefore, 

 from the eighteenth generation, all litters reared were off- 

 spring of parents that, according to the breeding records, did 

 not carry any of the mutant genes known to be present in the 

 strain. To facilitate the elimination of such genes, indi- 

 viduals in all litters reared in the twenty-first to the twenty- 

 fifth generations were inbred brother and sister. 



No new mutations appeared in the strain until the twenty- 

 second generation, when several rats, offspring of brother and 

 sister matings, were found to have more brown pigment in 

 their coats than is present in typical gray rats. Inbreeding 

 these rats gave young of which approximately one-fourth 

 were brown agouti, or cinnamon. This mutation had not been 

 reported previously in rats, although it was known in other 

 rodents (Castle, '31). Breeding tests showed that cinnamon 

 is a simple mendelian recessive, in which brown pigment re- 

 places black throughout the coat (King, '32). Crossing cinna- 

 mon rats ( AbCH) with pure blacks (aBCH) gave only normal 

 gray young. Inbreeding the Fj's from this cross gave non- 

 agouti brown or chocolate rats (abCH). 



Captive grays have bred true to type since the appearance 

 of the cinnamon mutants. However, two other variations 

 from the normal type have been found in mutant strains, 

 aside from the possible 'silver' mutation in black rats. The 



