178 Nature of the Genetic Material 



that DNA is the genie substance and the recent views of the con- 

 stitution of this molecule (Watson and Crick, 1953). According to 

 such views, a mutation would mean a change in the relative number 

 and order of purine and pyramidine rings (but the difficulty is that 

 the same type of process would have to account for mutation of the 

 gene and for differences between different genes). 



We have considered in an earlier section (I 3 B fl) the mutation 

 theory of McElroy and Swanson (1951), derived from the existence 

 of intermediate states and delayed mutation subject to a temperature 

 coefficient, and requiring the application of chemical kinetics to 

 explain the change of state by activation. This led also to a theory 

 of allelism which is essentially of the stereoisomeric type. The assump- 

 tion is that a gene, being a complex protein molecule, can exist in 

 many states differing in geometrical relationships and stability. Only 

 some of these can be recognized as different alleles, namely, if the 

 state is combined with a visible effect. The possibilities of such 

 stereoisomeric changes, directly or through an intermediate condition, 

 are almost unlimited. But they may be influenced or governed by 

 preferred intermediate states, the height of energy barriers, and the 

 interdependence or independence of different sites in a large molecule. 

 The hypothesis deals with an intramolecular pattern which is easily 

 changed without material change, and no difficulty will be found in 

 applying this hypothesis also to a supermolecular pattern. 



All these hypotheses, except possibly the last, require a gene 

 pair at a definite locus in the chromosome, and allelism is the 

 consequence not only of this identical location but also of the basic 

 chemical identity of a pair of alleles (except in the presence-absence 

 theory and its derivatives). Therefore, it should be a corollary of 

 this view that different genes cannot be allelic because they are 

 chemically different. However, if by chance or by origin through 

 translocation two genes at different loci were alike chemically and 

 thus could have mutants for which the same is true, they should 

 be allelic. Moreover, they should act as a series of multiple factors, 

 actually polygenes in the sense of Mather, and in combination show 

 additive and dosage effects. Furthermore, such polygenes would fall 

 by their origin under the definition of position alleles (Lewis) and 

 would have to exhibit all their features, though located in different 

 chromosomes. Actually, the so-called position alleles, assumed to be 

 the result of duplication (triplication, etc.) of one gene, fulfill only 

 one of these conditions: namely, behaving as alleles with no trace 



