306 THE CONTINUITY OF THE RACE 



which the last three groups cannot be distinguished phenotypically, and 

 that the genes concerned are C (dominant to c), the gene for chromogen, a 

 colorless color base; and E (dominant to e), the gene for an enzyme that 

 is capable of changing chromogen into a purplish red color. 

 This may be diagramed as follows: 



Parent varieties White X white 



CC ee cc EE 



F l CcEe 



Colored 



F 2 9 9 colored (CC EE, CC Ee, Cc EE, or Cc Ee) — chromogen, enzyme 



3 white (CC ee, or Cc ee) — chromogen, no enzyme 

 7 • 3 white (cc EE, or cc Ee) — enzyme, no chromogen 

 1 white (cc ee) — neither enzyme nor chromogen 



Here we have two sets of genes, neither able to produce a visible effect 

 alone but together producing a visible effect. Numerous other examples 

 of such complementary genes are known. 



Modifying genes — the 9:3:4 ratio. A very common type of gene 

 interaction is that associated with genes which do not themselves produce 

 any visible effect but which have the faculty of modifying the phenotypic 

 expression of some other non-allelic gene. Genes of this sort are very 

 common in the inheritance of the various coat colors of mammals. Among 

 the numerous domesticated varieties that have been developed from the 

 common mouse are a pure white (albino) and a pure black variety. The 

 "wild-type" mouse from which both of these were derived has a charac- 

 teristic gray color known as agouti, l a peculiar pepper-and-salt color that 

 is produced by each hair being banded with at least two colors, one basal 

 and one apical. 



If a black mouse is crossed with a certain type of albino, the F\ are 

 all agouti, and these, when mated among themselves, produce an Fi 

 that consists of approximately 9 agouti, 3 black, and 4 white mice. Here 

 the genes involved are B (dominant to b), a gene for black hair pigment; 

 and A (dominant to a), a gene that causes the pigment in the hair to 

 become localized and not evenly distributed throughout the hair. When 

 no pigment is present, the gene A can have no phenotypic expression. 



Parent varieties Black X white 



aa BB AA bb 



Fi Aa Bb 



Agouti 



F 2 9 9 agouti (AA BB, AA Bb, Aa BB, or Aa Bb) 



3 3 black (aa BB, or aa Bb) 

 ^3 white (A A bb, or Aa bb) 



white (aa bb) a new genotype for white 



1 The name of the color is derived from the agouti, a South American rodent in 

 which this type of coloration is particularly striking. 



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