202 FUNDAMENTALS OF CYTOLOGY 



The gene has been generally regarded as a niinute body with a con- 

 siderable degree of structural and physiological independence, perhaps 

 the last member of the series organism-cell-nucleus-chromosome-chromo- 

 mere-gene, the gene itself being possibly a single protein molecule or small 

 group of molecules. There have been various conjectures regarding 

 the nature of its chemical and physical activity, and attempts have 

 even been made to estimate its size. Most geneticists have, however, 

 been content to employ the gene concept mainly as a useful tool in 

 research and to define it, if at all, in terms of its effects, leaving the future 

 to furnish an adequate description of it in physicochemical terms. This 

 is a situation in biology comparable with that in physics and chemistry, 

 where the concept of the atom was long employed \\'ith conspicuous 

 success when far less was known about its actual nature than is known 

 today. The hope that our knowledge of the gene is to become more 

 intimate is encouraged by researches now in progress on the biochemical 

 aspects of genie action. In the ascomycetes, for example (page 167), 

 are nuclear conditions that are making it possible to associate particular 

 chemical reactions with certain genes, and from the nature of these 

 reactions it is expected that much can be learned concerning the physico- 

 chemical constitution of the genes involved. This association of cyto- 

 genetics with biochemistry promises to be as useful in leading us toward 

 a solution of the problem of the role of genes in ontogeny as the union 

 of genetics and cytology 40 years ago has been in elucidating their role 

 in heredity. 



Discussion of the nature of the gene has been stimulated anew by the 

 discovery that certain aberrations in the visible structure of the chromo- 

 some produce effects similar in many respects to those of gene mutations. 

 Three illustrative cases are the following. A color character, brown 

 midrib, which had been ascribed to 6m i, a recessive gene located near 

 the kinetochore in chromosome 5 of maize, has been found to develop 

 even when the region carrying this gene is removed altogether by a small 

 deletion. In Droso'phila the character roughest-3 mutated when the 

 small region carrying its differential gene in the X-chromosome was 

 inverted; moreover, when the former alignment was reestablished by a 

 reinversion, the normal character was restored. The dominant mutant 

 character, bar eye, in Droso'phila is now known to be due to a duplication. 

 This character may increase in intensity when a second duplication adds 

 still another like portion, and it may revert to normal when the extra 

 portion or portions are removed. An additional observation of impor- 

 tance is that in some X-rayed cells both gene mutations and chromosomal 

 deficiencies show the same response to variations in irradiation dosage, 

 the frequency of both varying as a linear function of the total energy 



