374 RADIATION BIOLOGY 



egg ("ootid") nucleus itself, while the dicentric is left outside. In this 

 way the egg comes to receive a noncrossover chromatid, which has an 

 equally good chance of being either a ring or a non-ring if the individual 

 had had both types to begin with. However, in some cases crossing 

 over occurs at more than one point in a tetrad, and, when this crossing 

 over happens to involve three or four of the chromatids of a tetrad, an 

 egg nucleus can be formed which fails to receive either a ring or a non-ring 

 derived from this tetrad. This result, leading to aneuploidy among the 

 offspring, is of course reproductively disadvantageous, although the 

 incidence of the disadvantage is considerably lower than that for most 

 translocations. 



When now we consider meiosis in the male, where all four nuclei 

 derived from each spermatocyte enter actual gametes instead of (as in 

 the female) being arranged in the form of one inner gamete nucleus and 

 three outer, polar body nuclei, it is evident that any crossover dicentric 

 chromatids must result in two deficient gametes, and/or gametes aborted 

 by bridge formation. Thus in the breeding of the male the reproductive 

 disadvantage conferred by a ring chromosome would in most organisms 



(including mammals) be high. How- 



ABCDEFGHI AB C D E F.G H I ^ • t^ , •, i 



■ ■ ' ever, in Drosophila and any other 



AB CDE FGHi AB CDEFG Hj_ gpecies (such as many Diptera) in 



ABEDCFGHi ABGFEDCHi which crossing over did not occur in 



(a) (J,) the male this disadvantage would be 



Fig. 7-5. Two breaks in one chromo- avoided. 



some, with all broken ends uniting so Another method of union of broken 



as to give rise to an inversion. In ends of a chromosome which has un- 

 (a) (on left) the inversion is paracen- ^ergone two breaks is for all three 

 trie, in (o) (on right) it is pericentric. , • , • ^ • • 



resulting chromosome pieces to join 



together again, with the middle piece still in the middle but facing in the 

 opposite direction, with regard to the end pieces, than that in which it 

 was before. This event, as well as the result, is called inversion (see 

 Fig. 7-5). It would be equally correct to describe it by saying that the 

 two end pieces interchanged positions with one another. If the centro- 

 mere is in one of the end pieces, the inversion is called 'paracentric; if in 

 the middle section, pericentric. In either case, all parts, including the 

 centromere, are present in the reconstituted chromosome, and its chro- 

 matids are properly transported to the daughter cells at each mitosis. 

 Hence it does not (except in the special case of "position effect," dis- 

 cussed in Sect. 9) result in damage to the descendant cells or individuals 

 which inherit it. 



However, when crossing over occurs between the chromosoine with 

 the inversion and its homologue of normal structure, within the region of 

 the inversion, aneuploid crossover chromosomes are produced, having a 

 deficiency of one region and a duplication of another. In the case of 



