THE MUTATED GENE 155 



frequently dosage differences between the two alleles. Facts 

 are therefore of interest that show that the action of the hetero- 

 zygote shows simple quantitative relations to that of the homo- 

 zygote. Many such cases have already been mentioned in the 

 chapters on rate genes and on multiple allelomorphs. One, in 

 which the type of reaction has been analyzed, might be added 

 here, viz., the pigmentation in the Himalaya rabbit. Here only 

 the "akra" of the body (tips of ear, nose, paws, tail) are pig- 

 mented according to a multiple allelomorphic series of colors. 

 W. Schultz (1915) proved that after plucking hair on the white 

 parts of the body, pigmented hair of the expected color is regener- 

 ated, if the temperature is decreased below 11°C. Iljin (1926) 

 found this temperature different for the different akra, and it may 

 be modified by experimental changes of skin temperature 

 (Schultz, 1929; Iljin, 1932) produced by cutting the sympathicus 

 or by hyperthyroidism. -Even different races with different 

 critical temperature were found. Engelsmeier (1935) showed 

 that these differences are based upon differences in skin tem- 

 perature and that the critical skin temperature above which no 

 pigment is formed is constant though dependent upon the 

 genotype, e.g., a n /a n , 33.2°C; a n /a, 30.6°C. This would mean 

 that the Himalayan gene produces a pigment-forming reaction 

 with a phenotypic alternative according to the skin temperature. 

 As the akra are normally the coldest points, this type of mosaic 

 pattern is the result of the alternative reaction norm together 

 with the outside conditions (cooling of akra). 



Schultz (1928) succeeded also in producing the blackening 

 effect in explants of skin. In this case, it is not the temperature 

 at the time of pigment formation that is decisive but the tem- 

 perature at the time of exposure before actual pigment formation. 

 It might therefore be assumed (Engelsmeier) that the oxidase is 

 formed only at low temperature and that its amount is dependent 

 upon the time of cooling as well as — with time constant — upon 

 the genes present (one or two quantities). Actually, with the 

 formula a n /a n , 5 min. in 25° sufficed for pigment formation in 

 explanations of white skin, whereas a n /a individuals needed 

 10 to 20 min. cooling. These experiments demonstrate as well 

 the alternative norm of reaction as its quantitative side as 

 correlated with the quantity of a process (probably enzyme 

 production) and its dependence upon the gene quantities. There 



