Chromosomes and Genes 179 



of dosage action. Thus none of these theories offer a very promising 

 explanation of the nature of allehsm. 



Our modern theory of the gene considers a mutant to be a 

 change of order or pattern on a supermolecular level within a section 

 of the chromosome of different, overlapping, and not strictly delimited 

 size. Since a position effect is, under this theory, the same as a 

 mutant and thus also as an allele, allelism means any deviation from 

 a normal sequence of parts within these overlapping sections; that 

 is, no chemical or quantitative change on the molecular level, but 

 rather a change in the arrangement of parts within an otherwise 

 identical field, the normal condition being that most frequent in the 

 species. 



Thus far we have considered allehsm only from the point of 

 view of substance and location, not of action. Actually, alleles have 

 a similar action which differs only quantitatively in the two alleles. 

 (The only apparent exception is the difference between the alleles 

 spineless-aristopedia in Drosophila, which requires an explanation 

 in terms of thresholds in order to fit it into the general scheme.) As 

 a rule, alleles in the heterozygote of a diploid organism work together. 

 This means that these alleles in the two different chromosomes must 

 produce something which reacts with the same substrate in such 

 a way that the two primary actions are additive or summative (this 

 includes also one positive plus one negative action) ceteris paribus 

 (no special modifiers). The result may vary (see below under dosage 

 and dominance) according to the threshold conditions of the entire 

 system. The resultant will be either half of the normal action, that 

 is, a pure dosage effect, or any condition between full effect (domi- 

 nance) and pure dosage effect. (This may be observed in the presence 

 of isoalleles; see below.) Within the classic theory we might think 

 that the action of alleles is a function of the gene and its nature. We 

 shall see, in the chapter on dosage effects, that a system of classifica- 

 tion of genes has actually been built upon this assumption. We think 

 that this is an erroneous conclusion. The genes (or, better, mutant 

 loci) are not different in regard to their general type of structure and 

 action. What is different is the developmental system which their 

 action primarily and secondarily affects, with all its special conditions 

 of substrate, thresholds, and velocities. 



But the possibility remains that the two alleles have a primary 

 action which is not at once submerged in the additive effect upon 

 the substrate. Moreover, it might remain independent for some time 



