90 



all cases agouti is dominant over the 

 absence of the pattern in blacks, 

 browns, etc. Onslow was able to ex- 

 tract a substance which inhibits the 

 oxidation of tyrosin by tyrosinase, 

 from the white belly of gray rabbits, 

 where the agouti factor removes all 

 black from the hair. Onslow compared 

 the case with the enzyme inhibitor 

 which he found in English rabbits but 

 on our interpretation, the latter was an 

 inhibitor of enzyme I, the former of 

 enzyme II. Since only one genetic 

 factor is involved, it seems likely that 

 the same cause which modifies a solid 

 black rabbit in such a way that yellow 

 ticking appears on the back is respon- 

 sible for the white belly. According to 

 the hypothesis presented here, white 

 appears on the belly when black is in- 

 hibited not because there is a general 

 inhibition of pigment production but 

 because enzyme I is below the thresh- 

 old for the appearance of yellow. 

 There is a parallel case in the white 

 bellied agouti mice. In this case, how- 

 ever, it is possible to increase the gen- 

 eral intensity of pigmentation in the 

 animal so that yellow appears on the 

 belly. In agouti guinea-pigs the belly 

 is normally yellow but paler than the 

 back. Such white patterns as those of 

 the gray rabbit and white-bellied 

 agouti mouse illustrate the possibility 

 of confusing white patterns due really 

 to yellow pattern factors of class 2a 

 acting where enzyme I is below the 

 yellow threshold with white patterns 

 of class la. It is likely that many dark- 

 eyed whites among mammals are of 

 the former kind. The polar bear is 

 probably an example.^^ 



It has been mentioned that the ex- 

 tent of white patterns seems to be 

 independent of the intensity of color 

 due to enzyme I. A parallel statement 



WRIGHT 



cannot be made of the agouti pattern. 

 The latter is greatly modified in extent 

 by conditions which have differential 

 effects on the production of black and 

 yellow. In pink-eyed guinea-pigs with 

 a much reduced potency of enzyme II, 

 the agouti band is greatly widened. On 

 the other hand, the agouti band in 

 ordinary intense guinea-pigs can be 

 greatly reduced by crossing with ex- 

 ceptionally intense blacks. Punnett's 

 density factor in rabbits ^^ eliminates 

 the agouti pattern altogether. It is also 

 more effective in the ordinary black 

 agoutis than in brown agoutis. Thus 

 the agouti factor seems to determine 

 a certain quantity of inhibitor which 

 is not as a rule sufficient to eliminate 

 all black and the effect depends not 

 merely on the level of the agouti fac- 

 tor but also on the level of potency or 

 quantity of the substance to be in- 

 hibited. The dominant agouti factor 

 (A) and the recessive factor of sooty 

 yellow rabbits (e) make an interesting 

 contrast between two factors of class 

 2a. In the former case, as we have seen, 

 Onslow demonstrated that an enzyme 

 inhibitor was produced, in the latter 

 he was simply unable to demonstrate 

 peroxidase, indicating a reduced quan- 

 tity or potency as compared with 

 blacks. The following table shows the 

 effects produced by these factors 

 when added to those of a solid black 

 rabbit (aaEE): 



21 Lang, A., 1914. Experimentelle Verer- 

 bungslehre. 



Back Sooty yellow Clear yellow 

 Belly Black White 



The case can readily be understood 

 if we suppose; first, that both en- 

 zymes I and II are strong on the back 



22 Punnett, R. C, 1912. Jour. Gen., 2. 



