INDUCED CHROMOSOMAL ALTERATIONS IN MAIZE 1301 



2. Internal Deficiencies. — These consist of the loss of a section of a 

 chromosome not inckiding an end. A very clear case (/>/ 9-2) has been 

 described by Creighton (21), in which the distal one-third of the short 

 arm of chromosome 9 was lost, but the conspicuous terminal knob was 

 retained. The deficiency involved the normal allelomorph of ygo. 

 The plant yielded no progeny. Another plant {Df 9-3), lacking the 

 normal allelomorph of yg-i, appeared only to have the terminal knob 

 slightly reduced in size. The plant was fully fertile. It was interpreted 

 as a very short internal deficiency including the base of the knob, but 

 another interpretation, that of mutation, is not excluded. McClintock 

 (30, 32) has described cytologically some further cases of internal defi- 

 ciency. The observed prophase figures usually involve a loop or foldback 

 in the one chromosome, or else a large amount of asynapsis. The 

 prevalence of nonhomologous pairing under these conditions makes the 

 chromosome configurations very irregular and difficult to interpret. 



3. Ring Chromosomes. — A number of cases have been described by 

 McClintock (30, 31, 32) of ring-shaped chromosomes or parts of chromo- 

 somes. These consist of the middle portion of a chrornosome with the 

 spindle attachment. Both ends have become eliminated and the ends 

 of the internal fragment united to form a closed ring. The cases studied 

 vary in size from large rings consisting of nearly the entire chromosome 

 to very small rings consisting of a very short section about the spindle 

 attachment. The type of meiosis pairing with a normal homolog is very 

 variable, usually involving much nonhomologous pairing. (McClintock, 

 32). Large rings usually pair in whole or in part with the normal chromo- 

 some. Small rings are more often completely separate from the normal 

 chromosome, appearing usually as a collapsed ring. The rings do not 

 remain constant in size but may become diminished or enlarged during 

 ordinary mitotic divisions. Occasionally groups of cells are found with 

 conspicuously smaller or sometimes larger rings. That these changes 

 involve eliminations or duplications of portions of the ring is shown 

 especially clearly in a large ring of chromosome 2 (McClintock, 31) 

 which included a conspicuous knob. This knob was missing in many 

 of the smaller sized rings. More rarely it was duplicated in large-sized 

 rings. Where the rings carry a dominant gene and the normal chromo- 

 some the recessive allelomorph, eliminations of the section of the ring 

 carrying this gene give rise to mosaic plants. 



The changing of size of the ring is undoubtedly related to the mecha- 

 nism of mitotic splitting of chromosomes, but the nature of the relation- 

 ship is not clear. If it were merely that the split was not oriented in 

 the same plane throughout the chromosome, the ring should sometimes 

 be doubled in size but not diminished. Interlocking rings would some- 

 times be formed and might be torn apart during anaphase separation. 

 From such fragmentation one would expect rod fragments rather than 



