180 Nature of the Genetic Material 



for eacli allele, vvhieh might mean a reaction first near, later away 

 from, the chromosomal site. In the extreme case of individual allelic 

 action, a condition would be reached as known for the blood-group 

 alleles or, possibly, for the sickle-cell anemia loci, where each allele 

 controls a different globin in the hemoglobin molecule (Pauling et 

 al, 1949). For the blood-group loci the heterozygote shows the 

 products (the agglutinogens) of both alleles. (But this is not the 

 entire story, as the quantity of agglutinogen, e.g., of A, is difiFerent 

 in A/B from that in A/O. ) 



It is probable that a comparable type of eflFect works in such 

 cases as the Sturtevant effect in Bar, where the presence of three 

 rearrangement breaks in one chromosome and the absence of breaks 

 in the other chromosome produce a quantitatively different position 

 effect from that with two breaks in each chromosome. The critical 

 problem of allelism is still far from a satisfying solution. But I 

 believe that the recent ideas on the nature of the gene will help 

 future work on the problem. 



d. Further members of the hierarchy 

 aa. Larger segments 



In the introduction to the present chapter I showed that the 

 theory of the organization of the genie material within the chromo- 

 some, independent of corpuscular genes, requires a complete hier- 

 archical order of active chromosomal fields up to and including the 

 entire chromosome. This means that the action of genie material in 

 the chromosome may be based at different moments upon any seg- 

 ment of a chromosome acting as an individual field, from a sub- 

 microscopic section below the size of a salivary chromosome band 

 to the sections just discussed, to larger sections of unified field 

 action which may be delineated or overlapping, to still larger field 

 units which finally may embrace the whole chromosome. If the 

 molecular structure of the chromosome, starting from the centromere, 

 is marked as 1 — > n, the functional units in the course of genie action 

 in development may be any possible fields, as indicated by the 

 braces in the diagram. 



Loci 123456789 10 11 12 -^n 



Segmenta 



Fields 2d order 

 Fields 3d order 

 Chromosome 



