1302 BIOLOGICAL EFFECTS OF RADIATION 



small rings unless the broken ends have an attraction for each other 

 sufficient to unite them even when they are not closely approximated. 

 Such a view seems highly improbable. 



Of the described ring chromosomes two arose spontaneously in plants 

 already carrying altered chromosomes. The balance resulted from X-ray 

 treatment. They were detected by means of the gene deficiencies 

 causing mosaic plants. 



INTERCHANGES 



Chromosomal interchanges or reciprocal translocations are frequently 

 found after X-ray treatment. The earliest discovered cases in maize 

 were found in untreated lines. These have figured most largely in the 

 literature, but interchanges from X-rays far outnumber those from 

 untreated sources. 



The genetics and cytology of chromosomal interchanges in maize 

 have been treated in a series of papers by Brink (6, 7), Brink and Burn- 

 ham (8), Brink and Cooper (9, 10, 11, 12), Cooper and Brink (20), 

 Burnham (13, 14, 15, 16, 17, 18), McClintock (28, 29, 30, 32, 33), Creigh- 

 ton (21), Creighton and McClintock (22), Rhoades (41, 44), Anderson 

 (1, 2), Anderson and Clokey (3), and Clarke and Anderson (19). Brief 

 reviews will be found by Sharp (46), and Anderson (2). In the latter 

 paper will also be found a catalogue list of all the interchanges described 

 for maize. 



In an interchange a fragment of one chromosome is exchanged with 

 a fragment from a nonhomologous chromosome. The two new chromo- 

 somes when homozygous behave in no way different from normals. 

 The two pairs of chromosomes involved may differ in appearance from 

 the normals and the corresponding linkage maps consist of differently 

 combined parts of the old maps. 



When heterozygous, the interchanged chromosomes synapse at 

 meiosis with homologous parts of the normal chromosomes, giving a 

 cross-shaped synaptic figure. The cross is at the point of interchange. 

 In the more typical cases, there are two long arms on opposite sides of 

 this cross, each with a spindle-fiber insertion, and two shorter arms 

 without spindle-fiber insertions. The two long arms are the main 

 portion of the chromosomes. The shorter arms are the interchanged 

 fragments and the homologous portions of the normal chromosomes. 

 For convenience we may designate the normal chromosomes as 1 and 2, 

 the interchanged ones as 1^ and 2^ Crossing over may take place in 

 any arm of the figure, as between 1 and the main portion of P or between 

 the interchanged fragment of P and the corresponding portion of 2. 

 During diplotene and diakinesis the synaptic figure opens out into a ring 

 usually with only the ends of the chromosomes attached. This ring 

 is best observed at diakinesis. The distribution at anaphase is ordinarily 



