Structural Changes in Chromosomes 



171 



Consequences of Two Breaks in Two 

 Chromosomes 



What happens when two breaks occur, one 

 in each of two different chromosomes? In 

 the first such case, the two broken chromo- 

 somes are nonhomologous (Figure 12-5). 

 If the two centric pieces unite, a dicentric 

 is formed and 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, then there is a mutual ex- 

 change of segments between nonhomologous 

 chromosomes, which is called a segmental 

 interchange, or more often, a reciprocal 

 translocation. This is the aneucentric type 

 of reciprocal translocation and often acts as 

 a dominant lethal in a subsequent division, 



particularly when the dicentric is pulled to- 

 ward both poles at once. 



The reverse is often just as likely, how- 

 ever; union occurs between the centric piece 

 o\' one chromosome and the acentric piece 

 of the nonhomolog, with the centric piece of 

 the second joining the acentric piece of the 

 first. This reciprocal translocation is of the 

 dicentric type. In individuals heterozygous 

 for such an exchange (Figure 12-6), hav- 

 ing two nonhomologs translocated and two 

 nontranslocated, gametes are formed with 

 deficiencies and duplications if, by segrega- 

 tion, they receive one but not both members 

 of the reciprocal translocation. 



When the chromosomes in nuclei are com- 

 pressed in a relatively small volume, no 

 broken end is far from any other; usually, 



FIGURE 12-6. Diagrammatic representation of segregation in dicentric reciprocal translocation 

 heterozygotes. (Chromatids not shown; the spindles — also not shown — have their poles oriented 

 vertically.) 



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SYNAPSIS 



F 



Zigzag Circle 



/ \ 



Open Circle 



DIAKINESIS 



AND 



METAPHASE I 



TELOPHASE I 



GAMETES 



ViV 



All Euploid 



vv vv 



All Aneuploid (Half-Translocationall 



