1274 BIOLOGICAL EFFECTS OF RADIATION 



larger problems of gene structure and behavior which underlie the whole 

 field of genetics. How does the chromosome in which a mutation has 

 been induced differ, physically or chemically, from an unaffected sister 

 chromosome, and how has this difference been brought about? 



The conventional picture of the material basis of germinal variation is 

 a simple one. Each chromosome includes a number of structurally 

 distinct elements, the genes, each of which plays a definite role in the life 

 of the cell and may thus have a specific effect on the inherent character- 

 istics of the individual. A germinal variation may be brought about in 

 either of two ways : (a) by a change in the constitution of a specific gene, 

 and (&) by the addition or removal of one or more chromosomes or chro- 

 mosome fragments containing genes. Changes of the first type (muta- 

 tions) are considered the sole source of qualitative differentiation of the 

 germ stuff; they represent the replacement of old genes by new and differ- 

 ent genes. Changes of the second type (chromosomal aberrations) are of 

 various types and may involve the loss or addition of a large or a small 

 number of genes. They are not directly concerned in the evolution of the 

 gene, but in many instances they produce phenotypic variations due to 

 change in the proportions of genes. Such variations are heritable, for 

 they are transmitted with the modified chromosomes. Various types of 

 variation due to chromosomal aberrations may be identified by char- 

 acteristic peculiarities of genetic behavior. Various environmental 

 factors have been found to influence the frequency of occurrence of certain 

 types of chromosomal aberration, but penetrating radiation was the 

 first which gave convincing evidence of an effect on gene mutation. 



If the appearance of a new mendelizing variation may be considered 

 proof of the origin of a new gene, there can be no doubt that irradiation of 

 the chromosomes leads to the production of new genes. But, though the 

 occurrence of a new gene may cause the appearance of a new mendelizing 

 variation, it is evident that mendelizing variations may occur also as the 

 result of other germinal changes. Any change in a chromosome which 

 permits cell survival and normal chromosome distribution will be trans- 

 mitted in the same way as a new gene. We have seen how, in polyploids, 

 even gross chromosomal aberrations may produce variations of a 

 mendelian type. Such spurious mutations may be detected by cytological 

 means, and in organisms very well known genetically, deficiencies or 

 duplications too small for cytological detection may sometimes be 

 identified by genetic tests. But no plant species is sufficiently well 

 known genetically to permit the systematic identification of deficiencies 

 by genetic means. The Drosophila evidence shows that segmental losses 

 may be of such small extent that the class of recognizable deficiencies 

 may overlap the class of apparent mutations. In plants it is only on the 

 assumption that deficiencies are invariably lethal to the gametophyte 

 that it is possible to justify the description of viable mendelian variation."- 



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