19-i 



FUNDAMENTALS OF CYTOLOGY 



When a deficient chromosome meets a nonnal one in synapsis, 

 homologous regions pair closely, leaving a region of the normal chromo- 

 some extending as an unpaired loop. Some degree of sterihty is expected 

 among the resulting spores or gametes. The great value of deletions in 

 determining gene location has already been emphasized (page 182). 

 Of special interest is the fact that they also reveal the presence and 

 position of unmutated genes which would not be detected in crosses of 



Fig. 139. — Diagram illustrating the production of a deletion (of the cd region) and the 

 synapsis of the resulting deficient chromosome with a normal chromosome. If the ring had 

 included the kinetochore (small circle), it would have remained functional and the ends of 

 the chromosome would have been lost. Such ring-shaped chromosomes disappear eventu- 

 ally becaxise of difficulties in mitosis. 



normal individuals, for in normal material both the normal gene and 

 its mutant allele must be present before the presence of either is suspected. 

 Inversion. — An inversion is a reversal in the position of a portion of a 

 chromosome. Most inversions are intercalary (Fig. 140); a few are 

 terminal. The mode of formation is apparently like that of deletions, 

 except that the four broken ends recombine in a different pattern. The 

 l^ehavior of a chromosome carrying an inversion is normal in mitosis. 



r /■ 



Fig. 140. 



-Diagram illustrating the production of an inversion (of the cd region) and the 

 synapsis of the chromosome carrying it with a normal chromosome. 



Its genetical effects are like those of an uninverted chromosome, except 

 for differences in linkage relations and, in some cases, a modified effect 

 upon characters due to the altered relative positions of certain genes 

 {position effect). Inversions can be used for the purpose of locating 

 genes when linkage relations \\dthin the chromosome are well known. 



At the time of synapsis in individuals heterozygous for the inversion, 

 the association of homologous regions results in the looped configuration 



