360 Action of the Genetic Material 



barred pattern behaves very similarly to the melanism of the nun moth 

 mentioned above. In both cases an explanation in terms of physiology 

 of development is in order. 



dd. Multiple factors and dosage 



A dosage phenomenon is certainly involved in multiple factor 

 inheritance. In the first example studied, Nilsson-Ehle's famous work 

 on color in oats, this is rather obvious, because duplicate and tripli- 

 cate factors are involved, supposed to be the same mutant loci multi- 

 plied by polyploidy. The effect appeared strictly proportional to 

 dosage in from one to six doses (= multiple loci). Hence the explana- 

 tion is obviously the same as in the bobbed case: each mutant locus 

 produces a certain quantity of something which is responsible for a 

 definite fraction of the effect. Thus the information on genie action 

 gained will be practically the same as in the dosage experiments. We 

 should assume that genuine multiple factors would act in the same 

 general way and thus lead to the same conclusion on dosage action. 

 Multiple factors are independently inherited mutant loci all of which 

 affect genically controlled phenotypic properties in the same way 

 (e.g., size). But this might mean very different things from the point 

 of view of genie action. In one group, dosage is certainly not involved. 

 For example, over-all growth of a vertebrate might consist of the 

 additive effect of growth of vertebrae, intervertebral discs, extremities, 

 and so on. Similar examples can easily be found. Certainly we cannot 

 speak here of a dosage effect of multiple factors. But when the indi- 

 vidual loci all affect the same chain of reaction, we might consider 

 this a dosage effect, though it can never be excluded that each locus 

 affects different components of such chains, while only the final effect 

 goes in the same direction. Our former example of melanism in the 

 nun moth may serve as an example. One sex-linked and two autosomal 

 dominants control the amount of pigmentation of the wing strictly 

 parallel to dosage (i.e., number of mutant loci present). Since we 

 may assume that production of melanin is a single series of synthetic 

 steps, starting with tyrosine or dopa (see Kikkawa, 1953), and always 

 the same within the same organism, a series of quantities of pigment 

 in proportion to the number of causative loci might mean a simple 

 quantitative influence of each locus upon the kinetics of such syn- 

 thesis. Actually, it turned out that one of the loci affects the pigment 

 between the bands of the dark wing pattern and the others the pig- 

 ment within the bands. The over-all effect is simply additive, because 

 it is based upon quantity of pigment, but the paths leading to the 



