CHROMOSOMAL MUTATIONS 



chains. X-ray diffraction studies showed that there are always two such 

 cliains spiraling about a common axis (Figure 87). Tlie two cliains are 

 united by liydrogen bonds between the bases. Analysis showed that equal 

 amounts of adenine and thymine or of cytosine and guanine were always 

 present, although amounts of adenine and cytosine, for example, might be 

 very unequal. They concluded that the bonds between DNA chains are 

 always formed either by adenine and thymine or by cytosine and guanine. 

 Thus there are four possible bondings between any two nucleotides: A-T, 

 T*A, C'G, and G'C. It is suggested that the sequence of these bonds could 

 encode unlimited genetic information, translatable by processes of develop- 

 ment into the great array of possible phenotypes, just as the simple dots and 

 dashes of Morse code can encode any verbal information. This theory 

 is not without defects, but it is one of the most exciting and productive 

 aspects of current genetics, and it tends to modify profoundly the older 

 concepts of the gene. 



One current theory of the gene envisions it as a more or less broad 

 field of function with a point focus on the chromosome, comparable to 

 the center of gravity of a physical object. The data of the corpuscular 

 gene theory, such as crossing over, deal with these point foci, while posi- 

 tion effects and pseudoalleles deal with the broader fields. These func- 

 tional fields may vary in size from molecular dimensions to the entire 

 chromosome, and they may overlap broadly so long as their foci remain 

 separate, as required by crossover data. This theory embraces elements 

 of both the divergent views of a few years ago, and it may well subserve 

 species formation upon lines related to both theories, and perhaps ac- 

 cording to others still unformulated. 



REFERENCES 



Darlington, C. D., 1937. "Recent Advances in Cytology," McGraw-Hill Book Co., 

 Inc., New York, N.Y. Badly out of date, but a classic of cytogenetics. 



Goldschmidt, R. B., 1940. "The Material Basis of Evolution," Yale University Press, 

 New Haven, Conn. The major statement of the evolutionary theory presented in this 

 chapter. 



Goldschmidt, R. B., 1955. "Theoretical Genetics," University of California Press, 

 Berkeley and Los Angeles. The last major work by this author. 



SwANSON, C. P., 1957. "Cytology and Cytogenetics," Prentice-Hall, Inc., New York, 

 N.Y. A very penetrating analysis of many of the problems touched upon in this 

 chapter. 



White, M. J. D., 1954. "Animal Cytology and Evolution," 2nd Ed., Cambridge Uni- 

 versity Press. The most complete presentation of this field now available. 



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