Structural Changes Within Chromosomes 



157 



gametes will be formed with deficiencies and 

 duplications if segregation results in their 

 receiving one but not both members of the 

 reciprocal translocation. 



In nuclei in which the chromosomes are 

 compacted in a relatively small volume, no 

 broken end is very distant from any other 

 broken end, and usually if one of the two 

 unions needed for reciprocal translocation 

 occurs, so does the other. This is the situa- 

 tion in the nucleus of the sperm of Drosophila 

 just after it has been involved in fertilization. 

 In oocytes, and probably in other cells that 

 have a relatively large nuclear volume, the 

 distances between the broken ends of non- 

 homologs are so great that reciprocal trans- 

 locations are comparatively rare, and even if 

 one cross union occurs the two other broken 

 ends usually fail to join to each other, so that 

 only half of a reciprocal translocation — • 



K L M N O 



1 • 



P Q R S T U 



called a half-translocation — is produced. 

 The loss or behavior of the unjoined frag- 

 ments usually causes descendent cells to die 

 or to be abnormal, as you would expect. 

 Half-translocations are sometimes found as 

 the result of segregation in heterozygotes for 

 a eucentric reciprocal translocation (see the 

 previous paragraph). Half-translocations 

 having this origin are known to occur in 

 human beings, for example. 



In view of the preceding discussion of trans- 

 locations, you can predict that such mutants 

 would tend to be eliminated from the popu- 

 lation. There is one exception to this, how- 

 ever, involving eucentric reciprocal trans- 

 locations in which both chromosomes are 

 broken close to their centromeres. In these 

 cases, almost a whole arm of each chromo- 

 some has been mutually exchanged. Such 

 whole arm reciprocal translocations when 

 heterozygous in Drosophila, and probably 

 most other species, undergo synapsis and 

 disjunction in such a regular manner that 

 euploid gametes (containing both or no pieces 

 of the translocation) are usually formed, so 

 that translocation heterozygotes of this type 

 are not at an appreciable reproductive dis- 

 advantage. 



K L 



M N O 



FIGURE 19-5. Reciprocal translocation between 

 nonhomologous chromosomes. 



ANEUCENTRIC TYPE 



Q R S T U 



K L Q R S T U 



P M_N O 



EUCENTRIC TYPE 



