HEREDITY AND VARIATION 



in the frequency of the appearance of mutations. It remained for H. J. 

 MuUer to show in 1925-1926 that X rays greatly increased the number of 

 chromosomal aberrations, as well as gene mutations. Other kinds of radia- 

 tion as, for example, radium emanations and ultraviolet light have been 

 shown to increase the rate of mutation. Some chemical compounds, such as 

 mustard gas, have been proved mutagenic. 



The manner of action of ionizing radiations, such as X rays, has been a 

 baffling problem. Evidence increasingly supports the concept of an indirect 

 eflfect rather than that of a direct hit by a particle on a gene target. Radia- 

 tion of a living cell produces its primary effect on the solvent water, dis- 

 rupting its molecules into H atoms and OH radicals; the H atoms may 

 combine with Og to form HOg radicals. These free radicals, OH and HOg, 

 are very active chemically and involve the compounds of the cell in unusual 

 reactions. Something which is produced as a consequence of these atypical 

 reactions acts as a chemical mutagen to modify the genetic material in some 

 way. The particular composition of the cell contents influences the response 

 to radiations. Thus radiations appear to produce their effects by indirectly 

 distorting the normal metabolic sequences of the cell. In germ cells the 

 genonemata become modified in such a way as to afTect the heredity of future 

 generations. 



It has been determined that the effects of ionizing radiations in producing 

 mutations are directly proportional to the dose. This is true whether a given 



Normal female 



Sperm 



AA 

 XY 



Normal male 



Superfemale 



I XXY ) 



Female 

 (both X's from mother) 



Oocytes 



Dis junctional 

 division 



Male 

 (X from father) 



0. 



Zygotes 



Dies 



Fig. 6.26. Diagram showing, in column I, normal disjunction of the X-chromosomes in 

 oogenesis and the subsequent possibilities of fertilization with typical sperm; in column II, 

 non-disjunction of X-chromosomes, both remaining in the ovum, and subsequent fertilization; 

 and in column III, non-disjunction of X-chromosomes, both passing to the polar body, and 

 subsequent fertilization. A is used as a symbol for a complete haploid set of autosomes. 



207 



