CHROMOSOMAL ABERRATIONS 203 



applied. The two processes are also similarly affected by dormancy 

 as opposed to activity in tissues at the time of irradiation. 



Such phenomena have led to a current theory that a gene is any small 

 portion of a chromosome having an effect upon character development 

 differing from that of neighboring portions, so that when this portion is 

 unlike in the two chromosomes of a pair Mendelian behavior is exhibited 

 by characters influenced by it. \ny change in this portion affecting its 

 influence, whether the change be a loss, gain, or rearrangeinent, represents 

 a mutation. In other words, genes are not all elements of the same 

 nature, even though their effects are generally comparable. The extreme 

 form of this view is that genes as discrete biological units do not exist 

 and that the only real genetical unit is the chromosome, in particular 

 the chromonema. This may be regarded as a sort of gigantic chain 

 molecule whose various parts alter the action of the whole in some man- 

 ner whenever they are sufficiently modified. Opponents of this extreme 

 view have cited phenomena in normal and aberrant material which 

 would not be expected in a single molecule. For example, the positions 

 of crossovers and induced breaks and the relatively small amounts of 

 energy required to produce them indicate the presence of numerous 

 distinct units not bound together by strong intramolecular chemical 

 bonds. They regard the evidence as indicative of a process of gene 

 mutation distinct from chromosomal aberrations, although both may 

 share in producing the phenomena studied by the geneticist. 



In any event, genes are localized constitutional conditions that can be 

 treated as units in genetical research. Such conditions are the physical 

 basis of Mendelian phenomena, and any stable modification of their 

 character-influencing power may be regarded as a mutation. We 

 continue to look to the future for a determination of the precise physico- 

 chemical nature of these conditions and the manner of their association 

 in the chromosome. In this search for further light on the nature of the 

 gene, few developments are more suggestive than the increasingly close 

 association of genetics, protein chemistry, and virus research. It seems 

 that genes, proteins, and viruses have much in common, and the clarifica- 

 tion of their relationships, when achieved, should be of immense value 

 in many branches of biological science. 



