114 Nature of the Genetic Material 



of a comparison. Just as most of the facts of genetics and especially 

 the elementary ones can be described in terms of the autonomous 

 gene, also the general facts of molecular constitution can be described 

 in terms of a dash for valencies. Just as the average geneticist dealing 

 with statistical consequences of Mendelism can do excellent work 

 without ever thinking of the nature of the gene, simply using the 

 primitive gene concept, it might never occur to the average chemist 

 to inquire into the nature of valency bonds. But if he really wants to 

 know what are the valencies with which he works all the time, he 

 must turn to the work on valency electrons and quantum mechanics, 

 which tell him that a valency is not a dash ( to put it crudely ) . In the 

 same way, the geneticist who meets with position effects and other 

 advanced subjects will be constrained to look into the old notion of 

 the gene and to find out whether its usefulness does not end at this 

 level, though he will not hesitate to teach elementary genetics in the 

 convenient terms of the classic gene, which are so useful for the 

 beginner, and, in addition, are all he needs for statistical genetics, 

 which, even to some geneticists, is the only genetics. 



b. Chromosome breakage 



We have discussed the basic fact of radiation-induced mutation, 

 the law of proportionality of mutation to dosage, without inquiring 

 into the nature of these mutants, though it was realized that sex-linked 

 lethals which Muller's technique made the basis of all such quanti- 

 tative work may frequently be deficiencies. Only after the introduction 

 of the salivary chromosome technique and cytological techniques for 

 discovery of chromosomal rearrangements in plant cells was it realized 

 that the ionizing radiations and X rays actually preponderantly 

 produce chromosome rearrangements, based upon breaks and their 

 abnormal union. Since that time, much work has been done to check, 

 separately, upon recognizable rearrangements in radiation experiments 

 (Muller, 1947, 1950fl; Kaufmann, 1941, 1948a; Sax, 1938, 1941; Catsch, 

 1948; Lea and Catcheside, 1942; Bauer, 1939, 1942; see also symposium 

 on chromosome breakage, 1953). 



It was found that rearrangements involving two breaks did not 

 follow the simple proportionality rule but approximately a quadratic 

 proportion, nearer to the exponent 1.5-1.7. The exponent 2 was of 

 course expected if two different hits were required. (The small devia- 

 tion was explained in different ways. ) As a check, single breaks after 

 radiation could be tested in some plant cells where they are visible, 

 and in ring chromosomes of Drosophila (Sax, Bauer). These gave a 



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