1168 BIOLOGICAL EFFECTS OF RADIATION 



makes investigations of chromosomal aberrations an important chapter 

 of genetics, even disregarding their evokitionary significance. The 

 various classes of chromosomal aberrations known at present were all 

 observed before the discovery of their artificial production by X-rays. 

 Chromosomal changes arise spontaneously in experimental cultures, and 

 at least some of them are encountered among individuals collected in 

 nature. But the frequency of their spontaneous origin seems to be 

 generally so low that it was not before Muller made his discovery of their 

 induction by X-rays that the field of chromosomal aberrations was really 

 opened for study. Among animals, chromosomal aberrations, spon- 

 taneous as well as induced ones, have so far been investigated largely in a 

 single organism, namely in the fly, Drosophila melanogaster. However, 

 comparable phenomena have been discovered recently also in other 

 forms, such as mice (Painter, 92; Snell, 125), grasshoppers (Nabours, 83; 

 Robertson, 114; Nabours and Robertson, 84; Helwig, 52) and, possibly, 

 in the parasitic wasp, Hahrobracon (Greb, 48). A comparison of chromo- 

 somal aberrations known in Drosophila with those in a variety of plant 

 species shows their profound analogy. The fundamental principles of 

 the structure of the chromosomal apparatus are similar in all living forms, 

 with the possible exception of bacteria and lower fungi. It is, therefore, 

 not surprising that the changes of this apparatus should be accomplished 

 by relatively few universally distributed methods. 



CLASSIFICATION OF CHROMOSOMAL ABERRATIONS 



The normal condition of the chromosomal apparatus in any strain, 

 variety, or species may be defined as one in which (a) the number of 

 chromosomes is constant, (&) every chromosome carries a definite set 

 of genes, and (c) the genes within each chromosome are arranged in a 

 constant linear series. Any deviation from the above regularities con- 

 stitutes a chromosomal aberration. We may adopt a modification of the 

 classification of chromosomal aberrations proposed by Bridges (17) and 

 Morgan, Bridges, and Sturtevant (71). 



ABERRATIONS INVOLVING SETS OF CHROMOSOMES 



Polyploidy. — Normal individuals carry in most of their cells every 

 chromosome in duplicate. Nondivision or fusion of nuclei in gameto- 

 genesis may result in the appearance in the following generations of 

 individuals having every chromosome represented three times (triploid), 

 four times (tetraploid), and so on. Just (54) induced polyploidy in 

 Nereis limbata by ultra-violet radiation. 



ABERRATIONS INVOLVING WHOLE CHROMOSOMES 



1. Polysomics are individuals having one of the chromosomes of the 

 normal set represented three or more times. 



