The Source of Variability 69 



responsible for the growth of highly specialized animals such as 

 whales with their massive structures and insects with their dif- 

 ferent and peculiar life history stages which together form the 

 ontogeny of a single individual. 



SIZE OF A GENE 



Little success resulted from efforts to calculate the possible size 

 of a gene by measuring the cross-over distances on highly organized 

 chromosomes such as those of Drosophila. The measurable units 

 were microscopic but even so were immense in terms of the sub- 

 microscopic units represented by even such large molecules as 

 protamines. More meaningful figures resulted from studies of viruses, 

 especially the phages. Studying inheritance in the phage known 

 as T4, Benzer (1955) found that each phage consisted of a simple 

 molecule of DNA, that abundant mutant alleles occurred in a spe- 

 cific region of this molecule, and that these mutants crossed over 

 readily. By calculating accurately the locus of each allele on the 

 DNA molecule, Benzer found that the average distance between 

 loci which crossed over was essentially identical with the calculated 

 length of one complete whorl of the DNA molecule, a distance 

 including about a dozen nucleotide pairs (see Fig. 17). Benzer 

 points out that these distances are suggestive only of the minimum 

 distances that might be involved in chemical changes of genie 

 material but that much greater distances may be involved in the 

 amount of genie material necessary to accomplish a specific function. 



DEFINITION OF THE GENE 



Because both the operation and the physio-chemical nature of 

 genes are unknown, probably the gene can be defined best in terms 

 of its effects. Efforts to formulate theoretical models or hypotheses 

 of genie action have resulted in highly divergent opinions of what 

 a gene is and how the term should be defined. 



The controversial views concerning the gene fall into two schools 

 of thought, one implying that specific molecules of the genie ma- 

 terial control each character (Dobzhansky, 1955; Beadle, 1955) 

 and the other that interactions between all parts of the genome are 

 responsible for the production of each character ( Goldschmidt, 

 1955). Stadler (1954) pointed out that the difi^erences of opinion 

 arise because one school defines the gene as an operational unit 

 whose action can be demonstrated by experiment, whereas the 

 other school defines the gene in a hypothetical physico-chemical 

 sense. 



