THE NATURE OF GENES I79 



for pink-eye, and this is indicated in the name **red-eye." 

 That the genes for red-eye and pink-eye are different in 

 chemical nature is shown by their complementary action. 

 When pink-eyed and red-eyed rats are crossed, black pig- 

 mented young result. 



A gene which in mice influences the action of Enzyme II 

 has the allelomorphic forms black (B) and brown (b). It is 

 not linked with the color gene and so cannot lie in the same 

 chromosome with it (Little and Phillips, Detlefsen). Gene 

 b interrupts the action of Enzyme II when the pigment has 

 been oxidized to a chocolate brown color, B allows the oxi- 

 dation to continue until the black stage is reached. 



Another gene which limits the action of Enzyme 11 is the 

 agouti factor. In mice it is not linked either with C or with 

 B. Hence it must lie in a third chromosome. It restricts the 

 action of Enzyme II to particular parts of the hair, the base 

 and tip of the hair in most body regions, and on the belly 

 the base alone, or it may exclude the action of Enzyme II 

 from the entire hair in the belly region. As the dominant 

 allelomorph of the agouti factor, the gene yellow inhibits the 

 action of Enzyme II more or less completely throughout the 

 coat of mice. 



In rabbits and guinea-pigs a gene called the extension 

 factor (E, e) influences the production or action of Enzyme 

 II. As E it permits black (or brown) pigment to be pro- 

 duced throughout the coat, except where its production is 

 interfered with by the agouti factor. As e, it does not permit 

 Enzyme II to function in the coat, but only in the eyes and 

 skin. Consequently the coat is yellow through the unas- 

 sisted action of Enzyme I. A third allelomorph, e', in guinea- 

 pigs allows Enzyme 11 to act in part of the coat only, thus 

 producing a yellow-and-black spotted coat. The extension 

 factor is apparently not linked with any of the other factors 

 for color production, and so must be located in a fourth 

 chromosome. 



How many other genes there are which influence the action 

 of Enzyme II, we do not know, nor do we know wluit their 



