376 RADIATION BIOLOGY 



7. STRUCTURAL CHANGES OF GREATER COMPLEXITY 



The larger the number of breaks, the higher is the probabiHty that 

 nonrestitutional union of the pieces will result in one or more acentric, 

 dicentric, or polycentric chromosomes and thus result in the death or 

 extreme abnormality of descendant cells or individuals. The euploid 

 changes (those involving neither deficiency nor duplication) that result 

 from just three breaks are the following: 



From breaks in three different chromosomes, (1) translocation by 

 triple exchange of a rotational nature, a piece of A becoming attached to 

 the "stump" (centromere-bearing fragment) of B, of B to the stump of 

 C, and of C to the stump of A. From two breaks in one chromosome and 

 one in another, (2) mutual translocation, with an inversion adjoining it 

 in one of the chromosomes, or (3) translocation of the nonmutual, deletion- 

 insertion type, a piece being deleted from chromosome A and inserted 

 into chromosome B at the point where B had been broken. From three 

 breaks in the same chromosome, (4) a result denoted as shift, involving 

 the interchange in position of the two interstitial fragments, each in the 

 place of the other, with or without the inversion of either one; this is, in 

 effect, deletion-insertion within one chromosome. 



Rearrangements have also been found which involve many more than 

 three breaks. Some of them are extraordinarily complex, but in all 

 cases they can be described as combinations of the various types of struc- 

 tural changes already set forth. 



In the case of all four of the euploid types of rearrangements resulting 

 from three breaks, offspring receiving them will ordinarily (barring 

 "position effect") appear normal, since there is no genie imbalance. 

 , However, in all cases, meiosis in the offspring carrying both the rearranged 

 chromosomes and (from their other parent) the normal homologues will 

 result in the production of some aneuploid gametes, the zygotes derived 

 from which will die ; thus these types will be at a reproductive disadvan- 

 tage, leading to their extinction. In the first three of the above four 

 types the aneuploid combinations will arise by means of the recombina- 

 tion of entire chromosomes, just as happens with ordinary translocations, 

 and the frequency of these aneuploids will usually be very high. In the 

 last type, the shift, the aneuploid combinations are formed by crossing 

 over, and their frequency will therefore be lower, depending mainly on 

 the length of the larger of the two pieces that were interchanged in posi- 

 tion. The rearrangements involving more than three breaks, like those 

 involving three, are almost always, when capable of surviving at all, 

 subject to reproductive disadvantages in later generations, and hence 

 these also tend to die out. 



In the discussion of structural changes involving more than one break, 

 only those types have been examined above in which the union of pieces 



