Til E Ml TA TED GENE 9 1 



The first Mendelian studies of animal pigmentation tried to 

 explain facts in chemical terms. Cuenot (1902, 1911) assumed 

 that albinos do not contain chromogens but oxidizing enzymes. 

 Later, Gortner (1910-1913) showed that the color varieties of the 

 potato beetle contained different amounts of chromogen in the 

 elytra. Onslow (1915), on the other hand, found that in rabbits 

 the main differences are not due to the chromogen but to the 

 enzyme component. He actually found peroxidases in the skins 

 of gray, black, and brown rabbits, which oxidize tyrosin to 

 melanin in the presence of hydrogen peroxide. In all recessive 

 white skins or parts of skins, he found the peroxidase missing, 

 though the chromogen was available; and in the dominant white 

 English, an antityrosinase prevents melanin formation. The 

 same was found in the belly of light-bellied pigmented forms. 

 In yellows, no peroxidase was found. 



Roller (1930) repeated the tests with the same results and 

 added that a pigmentation inhibiting extract from dominant 

 white does not act upon dominant black. (It ought to be added 

 that the relation found here cannot be generalized. In other 

 cases, genes for no or less pigment might control the chromogen 

 part of the reaction.) Wright (1916) used these facts — also 

 certain assumptions made by Little (1913) — to give a general 

 explanation of gene action in regard to coat color in chemical 

 terms. There is a basic color-producing enzyme (I) which 

 acting alone on chromogen produces a diffuse pigment which 

 appears yellow. The albino series of alleles produces this rather 

 unstable enzyme at different rates (see page 88). There is 

 another enzyme (also an oxidase) II, which stabilizes I and 

 makes it oxidize a chromogen into the coarser pigment granules of 

 dark pigmentation. I and II together set the threshold for 

 action of I at a lower level. The rate of II controls the admix- 

 ture of dark to yellow pigment in the different color types. 

 More details will be found later. Here belongs also the work of 

 Schultz (1935) on eye color of Drosophila which has been 

 discussed. 



The same problem has been attacked by Graubard (1933) for 

 pigment races in Drosophila. He tried to compare the tyro- 

 sinase content of yellow, wild, black, and ebony flies. To do this 

 he devised methods for estimating the enzyme concentration, 

 ceteris paribus. The results were very strange. Different 



