Inversion 397 



become joined and since it may be different in different cells, the 

 subsequent rings may be either larger or smaller than the original 

 and may differ from cell to cell. 



Inversion 



An inversion is an aberration in which one segment of a chro- 

 mosome has become inverted in position so that a chromosome 

 which was previously a b c d e f g hi j now has a changed struc- 

 ture such as a b c d g f e h i j. The e f g segment in the original 

 chromosome has become completely inverted in its position. 

 How both terminal and intercalary inversions are produced is 

 not entirely clear but, like deficiencies, they arise spontaneously 

 and can also be produced by various types of irradiation. The 

 intercalary type is far more common. 



AVe pointed out in Chapter 4 that the rule of strict part-by- 

 part pairing at zygotene holds even for a pair of homologous 

 chromosomes one of which has an inversion. The effect of this 

 rule is to throw the chromosomes into a loop which includes the 

 inverted segment and the noninverted corresponding segment of 

 the homologue (Fig. 17). This loop effect is also found in the 

 salivary gland chromosomes as the result of the pairing of iden- 

 tical bands (Fig. 104). 



One of the most interesting features of inversions is the result 

 produced when one or more crossovers occur between chromatids 

 within the inverted segment, perhaps best understood from Fig. 

 105. Since the chromosomal segments in the inverted region are 

 perfectly normal, they "split" into two chromatids, just as any 

 other regions of a chromosome ; and crossing over can take place 

 between any two of the four chromatids just as it can in any 

 noninverted region. Because of the inversion, however, the sub- 

 sequent results are different, and they are most striking at ana- 

 phase of the two meiotic divisions. 



If the inversion does not include the centromere and if one 

 crossover occurs within the inversion, two of the four chromatids 

 will be unchanged; the other two chromatids, however, will be- 

 come joined together in such a way that a long chromosome with 

 two centromeres and a much shorter fragment with no centro- 

 mere will be produced. At anaphase of the first meiotic division, 

 the long chromosome will be so oriented that the two centromeres 



