1298 BIOLOGICAL EFFECTS OF RADIATION 



are listed as inversions or inverted sections, though nothing is known 

 of their origin. One such inversion in the short arm of chromosome 8 

 (McChntock, 32) is present in a number of the strains used in genetic 

 studies. A short inversion in chromosome 4 including the spindle 

 attachment has also been described by McClintock (32). It causes an 

 apparent displacement of the position of the spindle attachment. A 

 similar displacement in chromosome 2 is sometimes observed and is 

 probably due to a similar inversion. 



From X-ray treatment, McClintock (30, 32) has described an inver- 

 sion in chromosome 2. Approximately two-thirds of the chromosome 

 became inverted. A similar inversion involving a still greater portion 

 of chromosome 2 has been found, also from X-ray treatment (Muntzing 

 and Anderson, unpublished). 



The longer inversions, when heterozygous, give typical inversion 

 loop configurations at meiosis (see McClintock, 30, Figs. 20-28, or 

 Sharp, 46, pages 315-318). Such figures are formed by the synapsis 

 of homologous portions in all parts of the chromosome. In short inver- 

 sions, loop configurations are seldom formed. Instead the inverted 

 sections are paired nonhomologously or else remain unsynapsed (McClin- 

 tock, 32). Homozygous inversions show normal synapsis. 



In the cases of long inversions considerable amounts of pollen and 

 egg sterility are found. This is due to crossing over taking place within 

 the inversion loop. When homozygous, fertility is normal. No studies 

 have yet appeared on the linkage relations in maize inversions, but the 

 linkage studies on Drosophila inversions together with McClintock's 

 work on nonhomologous pairing may serve as a basis for expectations. 

 Homozygous inversions should give normal linkage behavior, but with 

 the genes in different serial order. Heterozygous short inversions which 

 seldom show loop configurations, but do show nonhomologous pairing, 

 may show only a reduction or virtual elimination of crossing over in 

 the inverted section with no accompanying pollen or egg sterility. No 

 data are at hand to indicate if reduced crossing over beyond the limits 

 of the inversion is to be expected. Reduction of crossing over outside 

 of the limits of inversion occurs in Drosophila, but the cytplogical behavior 

 at meiosis is unknown. The best known of the Drosophila cases presum- 

 ably form loop configurations, since viable double cross-overs have been 

 found within the inverted sections. 



Heterozygous long inversions which consistently form loop configura- 

 tions should show partial pollen and egg sterility accompanied by a 

 reduction of observed crossing over due to elimination of most of the 

 cross-overs within the inversion loop. Except for rare cases where 

 unbalanced or deficient chromosomes may be viable, all single cross- 

 overs within the loop must be inviable. Crossing over may be further 

 reduced without additional sterility by nonhomologous pairing, incom- 



