704 TEXTBOOK OF ZOOLOGY 



One of the first problems to confront the radiologist was whether 

 nonkilling dosages would have any subsequent effects on the normal 

 cells. This was of interest especially in cases where the gonads had 

 been irradiated. Cases of temporary sterility have been reported 

 following roentgen ray treatment of the gonads and it was won- 

 dered whether the offspring of such treated individuals would de- 

 velop abnormally. Evidence has come in abundance from the gen- 

 eticists to the effect that the mutation rate of Drosophila is in- 

 creased to an amazing degree by irradiating the germinal cells. 

 If the gonads of either sex are irradiated, it is found that inbreed- 

 ing will bring out many recessive and harmful mutations in the 

 third generation (F-2). It should be pointed out that the danger 

 of harmful mutations is not of the same consequence in the human 

 because brother-sister matings are not permitted. As the mutant 

 genes are usually recessive, they would probably never be expressed 

 unless the practice were widespread enough to allow the mating of 

 two people both of whose parents had treated gonads. 



A contribution of greater theoretical importance than this warn- 

 ing to the x-ray therapist was the new method of increasing the 

 mutation rate. This addition of so many new characters in Drosophila 

 which would breed true allowed the geneticist to make rapid ad- 

 vances in supporting and clarifying the chromosome theory of in- 

 heritance. This has made possible the chromosome maps which 

 indicate the position of genes on the chromosome. Cytologists have 

 worked hand-in-hand with the geneticists, so that the approximate 

 location of the genes can be demonstrated visibly under the micro- 

 scope. 



The question naturally arose as to how the x-rays bring about 

 such changes in the hereditary material of the germ cells. It has 

 been found that most of the changes in the chromatin were so 

 minute as to defy attempts to demonstrate them morphologically. 

 Occasionally, however, the cytologist has found that the x-radiation 

 has caused parts of a chromosome to be moved from their normal 

 position. Moreover, in some irradiated cells a part of one chromo- 

 some has become attached to another. Other experiments which 

 have proved of great value to genetics have been concerned with 

 the action of x-rays in increasing the rate of crossing over and in 

 producing somatic mutations. 



