112 PHYSIOLOGICAL GENETICS 



account for such a combination of consistently quantitative 

 effects from different causes. 



We shall return to this case in the chapter on dominance in 

 allelomorphic series. 



C. Change of Dominance 



In early Mendelian literature, occasionally changes of domi- 

 nance within an individual were reported; e.g., in the cross of 

 red and yellow snails (Helix), the younger shells are yellow and 

 become red later (Lang, 1908). Many similar cases have later 

 found a different explanation or have not been studied suffi- 

 ciently. But there are some cases known that actually may be 

 described as a change of dominance. Goldschmidt (1917d, 

 19206, 19246) described such a case for the pattern of the cater- 

 pillars of Lymantria dispar. There is a race with bright-yellow 

 epidermal markings which are visible through the transparent 

 euticula and remain constant through all instars. There is 

 another race without these markings, except in very young 

 stages, in which the euticula is incrusted with dark pigment. 

 In Fi, the young caterpillars are light but, in the course of 

 development, become darker and are finally all pigmented. 

 This might be described as a change of dominance from light 

 to dark. What actually happens is that the cuticular pigment in 

 the hybrid is at first not present but slowly accumulates through 

 the instars, thus covering the underlying bright markings. 

 A comparison between the two races and the hybrid then shows 

 that the gene for pigmentation of the dark race controls a rate 

 of pigment production of such velocity that even the young 

 caterpillar is deeply pigmented. The allele present in the light 

 race controls a pigmentation process that is so slow that usually 

 it does not produce visible pigment. The heterozygous gene 

 (and also different alleles of the gene for pigmentation) controls 

 a rate of pigment production that is intermediate; therefore the 

 pigment covers the bright pattern only in later stages of develop- 

 ment, which then explains the phenomenon of change of domi- 

 nance in terms of rates of deposition of pigment. 



Another case that lends itself to a similar explanation has 

 been found by Honing (1923). He found a mutation of the Deli 

 tobacco called deformis which among other characteristics has 

 irregular narrow leaves endowed with excrescences on the lower 



