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CHAPTER 19 



zygote at a reproductive disadvantage. For 

 this reason, it is expected, after arising by 

 mutation, that only the smallest pericentric 

 inversions, which do not synapse when hetero- 

 zygous, will usually be able to survive in a 

 population. 



Following two breaks in the same chromo- 

 some, the last type of possible rearrangement 

 to be discussed is duplication (Figure 19-4). 

 If joining is delayed until after the broken 

 chromosome reproduces, the two interstitial 

 pieces may join, and, with the appropriate 

 end pieces, produce a eutelomeric chromo- 

 some with the interstitial region repeated 

 (neither, either, or both of these regions may 

 be inverted with respect to the original ar- 

 rangement). (The two remaining end pieces 

 may or may not join to form a deficient 

 chromosome.) Provided the duplicated re- 

 gion is small enough and acentric, such a 

 duplication may survive in the population. 



We shall consider now what is expected to 

 happen when two breaks occur, one in each 

 of two different chromosomes. In the first 

 case to be considered, the two chromosomes 

 may be homologs (ABCDEFG.HIJ). Usu- 

 ally, the breaks will be at different places, say 

 between A and B in one, and D and E in the 

 other. Exchange unions can occur one way 



to produce a dicentric and an acentric chro- 

 mosome, whose fate you can readily predict. 

 Other exchange unions can produce two 

 eucentric chromosomes in which BCD is 

 deficient in one and duplicated in the other. 



In the second case of this kind, the two 

 chromosomes broken are nonhomologous 

 (Figure 1 9-5). If the two centric pieces unite, 

 a dicentric is formed. The two acentric 

 pieces are lost in the next division, whether 

 they join each other or do not join at all. If 

 all pieces join as indicated, what is accom- 

 plished is a mutual exchange of segments 

 between nonhomologous chromosomes; this 

 is called a segmental interchange, or usually, 

 a reciprocal translocation, and is of the an- 

 eucentric type. This type often acts as a 

 dominant lethal in a subsequent division, 

 particularly when the dicentric is pulled 

 toward both poles at once. 



It is often just as likely, however, that union 

 occurs between the centric piece of one chro- 

 mosome and the acentric piece of the non- 

 homolog, with, vice versa, the centric piece 

 of the second joining the acentric piece of the 

 first. This reciprocal translocation is of the 

 eucentric type. In individuals heterozygous 

 for such an exchange, having two nonhomo- 

 logs translocated and two nontranslocated, 



BREAKAGE 



REPLICATION 



AB CDE FGHIJ 



AB CDE FGHIJ 



CROSS-UNION 



Figure 19-4. 

 Duplication {tandem type). 



ABCDECDE FG H 



A B 



F G H 



