Genetic Effects of Radiations 147 



as a mutation. A simple account of the physical principles in- 

 volved is given by Schrodinger.-' Probably not all changes pro- 

 visionally classed as gene mutations are intramolecular, but the 

 further consideration of this matter must be left until the grosser 

 effects of radiations on the chromosomes have been described. 

 Estimates of the sizes and of the number of genes may be 

 derived from mutation data. The best estimates are probably 

 those derived '^^^ '^^' ^" from a comparison of the mutation rates 

 induced by X-rays and neutrons. These two radiations differ 

 considerably in their relative efficiency in producing sex-linked 

 lethals, the ratio being about 1.6:1 for X-rtays : neutrons for a 

 given dose measured in terms of ionizations.^*^ This leads to an 

 estimated volume of a single gene of a1)()Ut 2.8 X 10"-" cubic 

 centimeters, containing about 1,000 atoms, and to there being 

 about 1,860 genes in the X-chromosomes of Drosophila, each 

 capable of giving X-linked recessive lethals. 



Induced Chromosoiiie Aberrations 



The chromosomes in a body cell pass through a cycle of divi- 

 sion, mitosis, whereby two nuclei, each an exact reproduction of 

 the parent nucleus, are produced. Before prophase, i.e., in the 

 resting stage, each chromosome divides lengthwise into two 

 chromatids, except at the centromere, and during prophase each 

 assumes a condensed spiral form and becomes coated with 

 nucleic acid. At metaphase, each chromosome moves on the 

 spindle so that the centromeres come to lie in the equatorial 

 plane. At anaphase, each centromere divides, the two halves 

 each with their attached chromatid then moving to opposite 

 poles of the spindle. A new^ nucleus is then organized at telophase 

 from each of the two groups of daughter chromosomes. 



Radiations affect the different stages in various ways. A 

 lengthening of the nuclear-division cycle may be caused, espe- 

 cially by heavier doses. A further physiological effect, shown by 

 adhesion or clumping of the chromosomes, occurs in cells already 

 in division at the time of irradiation.^-' ^^ With large doses, 

 excessive clumping may prevent the completion of mitosis. 



