NATURE OF THE GENETIC EFFECTS 363 



observation (involving, for example, Painter's salivary chromosome 

 methods in Drosophila and McCUntock's meiotic chromosome methods in 

 maize), combined with radiation techniques. 



The primary genetic event in structural change, regardless of the 

 nature of the causative agent or the type of chromosome structure 

 finally formed, has proved to be breakage of the chromosome thread. 

 This interpretation, which had been proposed as only one possibility by 

 the present writer (Painter and Muller, 1929), was first advocated by 

 Levitsky and Araratian (1931) on the basis of their studies on the plants 

 Crepis, Vicia, and Secale and by Stadler (1932) on the basis of his results 

 with maize. It was later supported by findings of McClintock (1932, 

 1938a, 1939) on the mechanical breakage, through entanglement, of ring 

 chromosomes in maize, by radiation dosage studies carried out on 

 Drosophila by Muller in collaboration with Belgovsky and others, using 

 genetic methods (Belgovsky, 1937; Muller, 1938, 1939b, c, d, 1940a; 

 Muller, Makki, and Sidky, 1939), and by Sax and his collaborators on 

 Tradescantia, using cytological methods (Sax, 1938, 1939; Sax and 

 Enzmann, 1939). 



The broken end of a chromosome thread, fractured either by ionizing 

 or other radiation, by mechanical means or by chemical mutagens (as in 

 the work of Auerbach and Robson), has the property of adhering to 

 another broken end when it meets it and, forming a permanent union, 

 thereby again constituting as continuous a thread as before, which is 

 capable of reproducing itself as such indefinitely. The most usual 

 broken end for the first one to meet is the other broken end derived from 

 the same break. In this case the combination formed is just like the 

 original unbroken thread, and the process is called restitution. 



If, instead of restituting at once, a broken end fails to join another one 

 before the chromosome reproduces to form two chromatids, then each 

 daughter chromatid fragment has a broken end like that of the mother 

 fragment, and both these broken ends have the property of adhesion. 

 The contact most likely to occur after that is between the homologous 

 broken ends themselves since they, just after their formation, must be 

 nearer to one another than to any other broken ends. In this way 

 chromosomes, called isochromo somes, consisting of two identical parts 

 joined mirror-image fashion, are formed (see Fig. 7-la-d). When these 

 fragments are not provided with a centromere — in which case they are 

 called acentric — their union produces an acentric isochromosome, while 

 union between the fragments which are provided with a centromere- 

 termed centric fragments — produces a dicentric isochromosome. It 

 sometimes happens, however, in a case in which a chromosome repro- 

 duces before it can undergo restitution, that two of the chromatid frag- 

 ments do later engage in restitutional union, while the others fail to meet 

 one another. In that case the centric fragment will be passed on down 



