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CHAPTER 19 



the two daughter nuclei are tied together, the 

 entanglement of the nuclei may interfere with 

 subsequent attempts at nuclear division, even 

 though, as in our example, the presence or 

 absence of the genes located in the bridge may 

 not be of paramount importance to the 

 functioning of these cells. 



Suppose, next, that the chromosome broken 

 is one of the large autosomes of Drosophila. 

 Detriment or death to one or both daughter 

 cells may occur because of the genes lost, 

 when either the acentric piece or the dicentric 

 fragment is left out of one or both daughter 

 nuclei. In addition, successive bridge-break- 

 age-fusion-bridge cycles may harm future cell 

 generations via the aneuploidy produced as 

 the result of the off-center breakage of di- 

 centric isochromosomes. It would be ex- 

 pected, other things being equal, that shorter 

 dicentrics would usually break and that 

 longer dicentrics would be more likely not to. 

 Of course, any bridge between nuclei that 

 does not break would be expected to have 

 the effect already mentioned. 



Single chromosome breaks may occur either 

 in the somatic or in the germ line. In the 

 latter case, aneuploid gametes may be pro- 

 duced. In the case of animals, since the 

 genes have been found to be physiologically 

 inactive in the gametes, aneuploid genomes 

 can enter the egg and sperm without impair- 

 ing their functioning. Accordingly, in ani- 

 mals, aneuploid genomes can be carried by 

 unaffected gametes into the zygote, which 

 subsequently may suffer dominant detrimental 

 or lethal effects. In most plants, however, 

 the products of meiosis (pollen, for instance) 

 perform certain physiological functions that 

 require the action of the haploid genome they 

 contain. So, in this case, aneuploidy is usu- 

 ally more lethal or detrimental before fertili- 

 zation than it is after. 



Having completed our discussion of the 

 consequences of a single nonrestituting break, 

 let us now consider the consequences of two 

 breaks that occur in the same chromosome. 



Such breaks can be located either para- 

 centrically, in which case both are to one side 

 of the centromere, or pericenin'cally, where 

 the centromere is between the breaks (Figure 

 19-2). 



Consider a chromosome linearly differ- 

 entiated as ABCDEFG.HIJ, the centromere 

 being between G and H. When the breaks 

 are paracentric in position (between A and B, 

 and F and G), the fragments may unite to 

 produce a centric chromosome (AG.HIJ, 

 Figure 19-2a) deficient for the acentric 

 interstitial piece (BCDEF). The ends of the 

 latter may join to produce a ring chromosome, 

 which, in any event, is lost in the next nuclear 

 division. When the breaks are pericentric 

 (between D and E, and H and I), the end pieces 

 are lost, whether or not they join together 

 (Figure 19-2c). The centric middle piece can 

 survive if its ends join to form a ring, and if 

 the deficient sections are not extensive. Even 

 if a ring can survive because it is not too 

 hypoploid (the aneuploid condition in which 

 genes or chromosomal regions are missing), 

 it is at a disadvantage in that a single chiasma 

 either with a nonring (called a rod) homolog 

 or with another ring results in a dicentric rod 

 or ring, respectively, as you can see by draw- 

 ing the configurations for these situations 

 yourself. 



Chromosomes with small deficiencies may 

 act as recessive lethals and may have a lesser 

 detriment than this when heterozygous ; those 

 with large deficiencies usually act as dominant 

 lethals in the next cell generation. Of course, 

 the nucleus, in which breakage or other events 

 occur which lead to a deficient chromosome 

 or, for that matter, any other structural 

 change, is still euploid. It is only after a 

 nuclear division that the daughter cells be- 

 come hypoploid or hyperploid (aneuploid be- 

 cause of an excess of genes or chromosome 

 parts). The preceding portion of this para- 

 graph should be reread with this considera- 

 tion in mind. 



It should be realized, in describing the 



