Color in Animals 313 



to magenta or rose, whereas gene adds two such groups, chang- 

 ing the color to blue or mauve. Thus it is seen that the more 

 the hydroxyl or methoxyl groups, the bluer the color. It is also 

 seen that genes can act by bringing about chemical substitutions 

 in a complex organic molecule. Another gene, D, brings about 

 a further change, causing hexose molecules to be substituted at 

 positions 3 and 5 on the pigment molecule, and in dd plants a 

 hexose and pentose molecule is at position 3 and no substitution 

 at 5. Structural formulae for three of these types are shown 

 in Fig. 87. 



HO 



/V^- 



OCH^ 



OH HO 



OCR, 



O- hexose \^ \/ O- hexose Y ^^ O- hexose- pentose 



O- hexose O- hexose 



Blue Magenta Salmon 



Fig. 87. Structural formulae for three anthocyauin types in Strepto- 

 carpus. (Redrawn from Lawrence, Scott-Moncrieff, and Sturgess in the 

 Journal of Genetics.) 



Color in Animals 



As in plants, enzymes appear to play an important role in the 

 determination of color in animals. The black pigments of ani- 

 mals belong to a group known as the melanin pigments, which 

 result from the reaction of a chromogen, such as tyrosin with 

 an enzyme. If either the chromogen or the enzyme is missing, 

 the pigment does not develop. One of the earlier studies shows 

 that in rabbits the presence or absence of color depends upon 

 the enzyme, for the chromogen is present even in albinos. Ons- 

 low found that in dark-colored animals enzymes of the type 

 known as peroxidases were present which react with tyrosin to 

 produce the melanin pigment; in albinos this enzyme was absent 

 and the pigment could not be formed. Wright has postulated 

 two enzymes to explain pigment in animals in general. Both 

 these enzymes can oxidize chromogen, but they act in different 

 ways. He assumes that the chromogen is present in all indi- 

 viduals. If one of the enzymes is present (enzyme I), the 

 chromogen is oxidized and a yellow pigment is formed. If the 

 second enzyme is also present (enzyme II), it interacts with 

 enzyme I and the two together react with the chromogen to 



