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CHAPTER 21 



tions occur in these Y-containing arms, the 

 rod shapes will be attained, completing the 

 change from a V to two rods. Note that al- 

 most all but the centromere of the Y chro- 

 mosome is eventually lost in this process. 

 But this may have little or no disadvantage 

 since the Y carries relatively few loci and is 

 primarily concerned with sperm motility. 

 For example, this series of mutations may be 

 initiated in the male germ line so that two 

 Y-containing rods are produced. Deletion of 

 Y parts can occur without detriment if these 

 chromosomes happen to enter the female 

 germ line, or they may stay in the male germ 

 line provided a regular Y chromosome is 

 added to the chromosome complement in 

 due time. The small IV chromosome in 

 melanogaster, whose monosomy is tolerated 

 in either sex, may also serve to contribute a 

 centromere in changing a V to two rods by an 

 identical or similar series of mutational 

 events. 



The metaphase plate observations confirm 

 expectation in the case of Drosophila (Chap- 

 ter 20), in that whole arm translocations are 

 capable of persisting in natural populations. 

 Such rearrangements and pericentric inver- 

 sions are very useful in helping us establish 

 evolutionary relationships among different 

 species. But it should be emphasized that 

 this kind of information by itself does not 

 prove either that the formation of different 

 species is a primary consequence of the oc- 

 currence of these rearrangements, or that these 

 rearrangements are of secondary importance 

 in species formation, or that these mutational 

 events occur after species formation is com- 

 pleted. As exemplified by Oenothera and 

 Drosphila, we have seen that gross rearrange- 

 ments of somewhat different types have per- 

 sisted in the course of the evolution of dif- 

 ferent groups of organisms. For this reason 

 it would be prudent to refrain from predict- 

 ing, except in the general way we have done 

 in Chapter 20, which, if any, structural 

 changes would be found associated with the 



evolution of other particular groups of or- 

 ganisms. 



Let us turn our attention now to the factors 

 which produce the breaks leading to struc- 

 tural change. Chromosomes do break spon- 

 taneously; that is, they occasionally break 

 during the normal course of events. How- 

 ever, because spontaneous breaks occur rela- 

 tively rarely, the study of them and their con- 

 sequences would be greatly enhanced by the 

 application of external agents capable of pro- 

 ducing breaks in great numbers. One of 

 these agents is radiation, and our attention 

 will be restricted to this agent in the present 

 Chapter (Figure 21-4). The process of break- 

 ing a chromosome is a chemical reaction 

 requiring energy. Radiation may supply 

 energy in three different forms: heat, activa- 

 tion or excitation, and ionization. The bio- 

 chemical effect of a radiation depends upon 

 the type and amount of energy it leaves in 

 tissue, less energetic radiations (like visible 

 light) leaving energy in the form of heat, more 

 energetic radiations (like ultraviolet light) 

 leaving energy also in the form of activations 

 (in which an electron is moved from an inner 

 to an outer orbit). The more energetic the 

 radiation, the greater is the likelihood that 

 the energy absorbed will lead to chemical 

 change. For example, visible light does not 

 produce as many breaks in chromosomes as 

 does ultraviolet light. Radiations of still 

 higher energy (like X rays, gamma rays, and 

 alpha, beta, neutron, proton, and other fast- 

 moving particulate radiations) are most likely 

 to cause breaks. Although such high-energy 

 radiations can heat and activate, most of their 

 energy is left in cells in the form of ioniza- 

 tions, and it is these which produce most of 

 the breaks. Let us consider first what ioniza- 

 tion is and how it is produced, and then how 

 it is connected with the production of breaks. 



X and gamma rays are electromagnetic 

 waves like visible light, but have relatively 

 shorter wave lengths, and can penetrate 

 tissue more deeply before they are stopped, at 



