156 GENETICS 



tive inheritance in which the mosaic appearance is 

 caused by a Mendelian determiner, in this instance a 

 spotting gene or, in other words, the absence of a gene 

 for uniformity. 



Miss Durham, in her work with mice, has demon- 

 strated an intensifying gene, the absence of which she 

 calls a diluting gene. The action of the former pro- 

 duces, as its name implies, intensity of color, while that 

 of the latter serves to lessen the degree of intensity in 

 which color appears. 



These genes of intensity and diluteness, it should 

 be observed, do not in any way correspond to the 

 duplex and simplex condition of a dominant color 

 character, either of which would straightway appear 

 if crossed with an albino. The factors of intensity 

 and dilution of color are of an entirely different na- 

 ture, as they have been proven to be independently 

 transmissible through albinos where a color character 

 could not appear because of the absence of pigment. 



The following illustration of this kind of supple- 

 mentary genes taken from Miss Durham's experiments 

 will serve to make the case clear. The symbols em- 

 ployed are: 



B = black pigment which masks brown, or chocolate. 

 6 = the absence of B, consequently chocolate. 

 I = intensity gene. 



i = dilution gene or absence of intensity. 

 C a complementary color gene acting with P. 

 p = a complementary pigment gene acting with (7. 

 BICP = black. 



