NATURE OF THE GENETIC EFFECTS 447 



has some of its parts (notably the blocks) very differently spaced from 

 the extended (salivary or other interphase) chromosome.^ 



The wealth of chromosomes with parts variously rearranged by radi- 

 ation was further amplified by genetic techniques whereby, through 

 crossing between the different chromosomal mutants, still more diverse 

 recombinations, having given parts missing or duplicated, were obtained. 

 Through the cytogenetic study of the resulting material it was then possi- 

 ble to gain information concerning the properties of the different chro- 

 mosome parts. In this way it was proved that the centromere is the 

 product of a gene, or a minute group of genes, located at the same point 

 as that at which the centromere itself appears, but capable of being 

 altered in position by structural change along mth the chromosome 

 region surrounding it. It was likewise established that one centromere 

 is necessary for a chromosome to be transported at cell division, but that 

 more than one, even if they are close together, leads, in the material used, 

 to loss of the chromosome, and that chromosome bridges caused by dicen- 

 tric chromosomes in fertilized eggs usually result, in Drosophila, in the 

 death of the zygotes. Similarly, evidence was obtained of the genetic 

 permanence of the telomere and of its being due to a gene or a minute 

 group of genes located at the point at which it appears. 



Other differentiated chromosome parts that were shown to be produced 

 in situ in Drosophila (i.e., by specific genes located at the points in the 

 chromosome at which the given structures appear) and to be separable 

 by structural change both from each other and from the centromere, are 

 the blocks (bodies whose existence had not previously been suspected), 

 the nucleolus, and minute regions containing what may be called "conju- 

 gator genes," which give those regions a powerful effect on chromosome 

 conjugation and segregation. The specialized genes responsible for all 

 three of these kinds of effects were found to be congregated together, 

 along with the centromere, in a given chromosome region, the main 

 heterochromatic region, which exhibits a whole complex of other char- 

 acteristics. These include, as was shown by these investigations, heter- 

 ologous conjugation, relative dispensability of genes, susceptibihty to 

 structural change, characteristic cytological appearance, and peculiar 

 type of position effect. Studies of rearranged chromosomes showed that 

 these characteristics also were all in situ results of the genes in the same 

 regions, and that they were not to be attributed just to the specialized 

 genes above mentioned, which formed but a small part of these regions, 

 3 Except in certain special cases, where the sources of evidence are not well known 

 even to many geneticists, references will not be given to the voluminous literature in 

 which the production of genetic changes by radiation has been used primarily as a 

 tool in the investigation of genetic or other biological problems themselves, since the 

 main purpose of the present chapter is to acquaint the reader with the nature of the 

 genetic effects of radiation, from the point of view of those interested in radiation 

 effects as such. 



