I 



XLII 



INDUCED CHROMOSOMAL ALTERATIONS IN MAIZE 



E. G. Anderson 



William G. Kerckhoff Laboratories of the Biological Sciences, California 

 Institute of Technology, Pasadena, California 



The normal chromosomes. Inversions. Deficiencies. Interchanges. Nature of 

 the alterations produced by irradiation. References. 



THE NORMAL CHROMOSOMES 



The great majority of varieties of maize grown in the United States 

 appear to be uniform in the arrangement of material in the chromosomes. 

 This arrangement we accept as the normal. The 10 normal chromosomes 

 of maize are figured diagrammatically by McClintock (32, page 192). 

 Different varieties or strains present such visible differences as presence 

 or absence of given knobs or differences in size, shape, or appearance of 

 given chromomeres and knobs. Two types of satellite on chromosome 6 

 have also been described (McClintock, 33). 



In addition to the normal diploid set of 10 pairs of chromosomes, 

 there may be present in some strains, one or more j5-type chromosomes 

 (Randolph, 36, McClintock, 32). These are an aberrant type of chromo- 

 some, behaving somew'hat less regularly at meiosis and having no appar- 

 ent function in inheritance. 



The 10 normal chromosomes ordinarily synapse regularly at meiosis, 

 but infrequently some portions of the chromosomes may appear paired 

 nonhomologously. Such nonhomologous pairing, although rare in normal 

 diploids, is frequent in many cases of unbalanced or altered chromosomes 

 (Burnham, 15 and 16, McClintock, 32). A thorough study of non- 

 homologous pairing has been made by McClintock (32). 



Randolph (38, 39) has reported tetraploids induced by temperature 

 treatments. Losses of a chromosome induced by X-rays have been 

 detected cytologically (Randolph, 37). Some of the induced gene 

 deficiencies detected by endosperm characters (Stadler 48, 50, 52) are 

 probably whole chromosome eliminations. Increases or decreases in 

 chromosome number will not be treated further except to be included 

 in the bibliography. 



INVERSIONS 



In a few of the strains of maize which have been studied cytologically, 

 a portion of a chromosome appears in a reversed order. These cases 



1297 



