CHAPTER XIX 

 RECIPROCAL TRANSLOCATION 



The mutual exchange of portions of chromosomes known as reciprocal 

 translocation, or often as "segmental interchange," differs in a number 

 of respects from that occurring in ordinary crossing-over. In crossing- 

 over, two homologous chromatids of a tetrad exchange portions which are, 

 with rare exceptions, exactly equivalent; and this is done with character- 

 istic frequencies in most of the diploid organisms which have been 

 adequately studied. On the other hand, in reciprocal translocation, 

 which is of relatively rare occurrence, the chromosomes involved may be 

 either homologous or non-homologous, and the exchanged pieces may be of 

 any relative size. When the exchange is between homologues and 

 involves approximately equivalent pieces, the effects may resemble those 

 produced by crossing-over; but ordinarily, especially when non-homo- 

 logues are involved, the cytological and genetical consequences of 

 reciprocal translocation are highly characteristic and not to be confused 

 with those of normal crossing-over. Just how such translocations are 

 accomplished is not yet known, but processes like those supposed to 

 bring about simple translocation are suggested. Moreover, it is not 

 improbable that many supposedly simple translocations are actually 

 reciprocal, one of the exchanged pieces being missed because of its 

 minuteness and the consequent failure to form the chromosome rings 

 characteristic of translocations of the latter type. Just such a case has 

 been described in Zea (Burnham, 1932). 



Origin of the Theory. — In Datura stramonium, Blakeslee and his 

 associates^ found that any one of the 12 chromosomes might be present 

 in triplicate in the somatic cells, giving 12 visibly different "primary 

 mutants" with 25 chromosomes instead of the normal 24. The primary 

 type due to the presence of an extra chromosome I (the longest of the 

 set) was called "Rolled." When Rolled plants were bred there appeared 

 among the offspring four types: (1) normal plants without the extra 

 chromosome, due to the fact that half of the spores of Rolled do not 

 contain it; (2) Rolled plants carrying the extra chromosome; (3) plants 

 of a "secondary" 25-chromosome type called "Sugarloaf"; and (4) 

 another secondary type called "Polycarpic." Other primary types also 

 were found to have their corresponding secondaries. The secondaries in 



1 For general accounts of the phenomena treated here, see Blakeslee (1928, 1929o, 

 19316). See also Belling (19276cd, 1928/) and Bergner et al. (1933). 



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