INDUCED CHROMOSOMAL ALTERATIONS IN MAIZE 1299 



plete synapsis or other causes. There is need at the present time for 

 adequate Unkage studies on inversions of known cytological behavior. 



The inversions found in our genetic stocks were picked up during 

 the course of cytological examinations for other purposes. The inversion 

 in chromosome 2 from X-ray treatment (McClintock 30, 32) was similarly 

 discovered in the course of cytological examination of a deficiency in 

 another chromosome. The unpublished long inversion in chromosome 2 

 (Muntzing and Anderson) was detected by means of partial pollen 

 sterility. No data are available to indicate the frequency of occurrence 

 of inversions following irradiation, but it is probably high. The only 

 ones readily discovered are the long inversions which may be detected 

 and followed by means of the partial sterility of pollen. 



DEFICIENCIES 



1. Terminal Deficiencies. — These consist of the loss of an end of a 

 chromosome. Several cases have been described by Stadler (50) and 

 McClintock (30). These were found after X-ray treatment and were not 

 transmitted to the following generation. In each case the zygotes 

 deficient for a portion of a chromosome were viable and vigorous, but 

 the spores with a deficient haploid complement did not survive to func- 

 tion. These deficiencies were found by placing pollen carrying dominant 

 genes on silks of the appropriate recessives and irradiating during the 

 growth of the pollen tubes. Plants deficient for a dominant gene were 

 examined cytologically at meiosis. A portion of one chromosome was 

 missing. The deficient chromosome paired normally with the homolo- 

 gous portions of the corresponding normal. Beyond the end of the 

 deficient chromosome, the normal chromosome appeared as a single 

 unpaired thread. By the identification of the chromosome involved 

 and by measurement of the region deficient, it was possible to assign 

 certain genes* to definitely limited regions of particular chromosomes. 



More cases of terminal deficiency from X-rays have been described 

 by McClintock (32), Stadler (53), and Creighton (21). Several of these 

 cases are of especial interest. Stadler's (53) study of a deficiency for 

 the gene R showed the absence of the terminal one-fifth of the long arm 

 of chromosome 10. Deficient microspores gave rise to small-sized 

 immature-looking pollen grains which were unable to function. Eggs 

 carrying the deficiency functioned about one-third as frequently as the 

 normal. Plants heterozygous for the deficiency were less vigorous and a 

 little later maturing than normal plants. 



Creighton (21) has described three deficiencies for yg2 at or near the 

 end of the short arm of chromosome 9. One of these {Df 9-1) was defi- 

 cient for about one-fourth of the arm including the conspicuous terminal 

 knob. The absence of the knob is cited as proof that the deficiency is 

 really a terminal one. 



