200 



AN INTRODUCTION TO MODERN GENETICS 



underlying pattern of thresholds is the Himalayan rabbit/ which is 

 white except for black patches on the paws, nose and tail. The gene 

 concerned is an allelomorph of the albino locus. Its action is to deter- 

 mine a pattern of temperature thresholds above which the formation of 

 tyrosinase is impossible. Over most of the body the threshold is such 

 that the body temperature is too high to allow the enzyme to be pro- 

 duced, so that the skin remains white. If the animals are kept at a low 

 temperature, pigment may be formed. In the exposed regions at the 



X 



Fig. 94. Variations In the Manifestation of the Gene vti in Drosophila 

 Funebris. — Above to the left is a normal wing, and beneath it three rows of figures 

 showing the manifestation of vt/ in three different inbred stocks. In each row the 

 degree of expression (expressivity) increase from left to right. At the right are 

 three diagrams of the transverse vein, which is drawn thinner the more readily 

 it disappears in the stock concerned. 



(From Timofeeff-Ressovsky.) 



extremities the temperature of the skin is lower and the enzyme is 

 formed. The threshold in these regions is also lower, since if body skin 

 is transplanted to the extremities it does not produce black hair. 



In some cases the underlying pattern of thresholds can be modified 

 by selection. Timofeeff-Ressovsky^ has described different inbred stocks 

 of Drosophila funebris which all carry the gene v^^ (venae transverae 

 incompletae) but in each of which it has a particular type of manifesta- 

 tion (or specificity). In one line, the feebler grades of expression of the 

 gene have a break in the lower end of the transverse vein, in another 

 line the break occurs at the upper end, while in the third the vein may 

 be broken at both ends. In all three lines the higher grades of expression 

 lack the vein altogether. Timofeeff-Ressovsky considers this in terms of 

 a "vein-breaking" stimulus and the resistance of the vein; one might 



* Cf. Engelsmeier 1935, Danneel 1938. ^ Timofeeff-Ressovsky 193 1. 



