CHROMOSOMAL ABERRATIONS 



195 



shown in the diagram. Meiosis may be carried through, yielding spores 



or gametes with and without the inversion. Frequently, however, there 



is sterihty, sometimes in a high degree. This is due in part to the 



disturbing effects of crossing over in the region of the 



inversion, for this may produce a chromatid with two 



kinetochores and a chromatid with none. The 



akinetic chromatid is lost, while the dikinetic one 



either forms a "bridge" between the two groups at 



anaphase (Fig. 141) or goes in its entirety to one pole. 



In either case an abnormal complement and sterility 



result. 



Such sterihty is of special interest in connection 

 with the origin of diveree types in nature, for it has 

 l)een found that certain geographical races of Droso- 

 phila differ in being homozygous for different inver- 

 sions. These races are meiotically regular and fertile, 

 but heterozygotes formed by crossing two of them are 

 not; hence, two such races are kept distinct bj^ this 

 internal mechanism and can proceed independently 

 with their differentiation into more widely divergent 

 types. 



Translocation. — The transfer of a portion of one 

 chromosome to another except by normal crossing over 

 is known as translocation. It may involve homologous 

 or nonhomologous chromosomes. In nearly all cases 

 the translocation is reciprocal, i.e., parts of two chromosomes are 

 exchanged (Fig. 142). These parts may be of any relative length, 

 for breaks and recombinations evidently can occur at many places in 

 the chromosomes. Sometimes one of the resulting chromosomes has 



Fig. 141.— Ana- 

 phase / in inioro- 

 .spororyte of maize 

 showing a chromatid 

 bridge due to cross- 

 ing over in the region 

 of an inversion. 

 The crossover re- 

 sulted in a clu-omatid 

 with two kineto- 

 chores and one with 

 none. The latter is 

 seen as a small frag- 

 ment near the bridge. 

 {Courtesy of B. Mc- 

 Clintock.) 



Fig. 142. — Diagram illustrating reciprocal translocation between nonhomologous chromo- 

 somes and the synapsis of the resulting chromosomes with their normal homologues. 



two kinetochores while the other has none, but in the successful cases 

 each translocated chromosome has one only and behaves normally in 

 mitosis. The simple translocation of one piece without exchange seems 

 to occur very rarely. 



