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CHAPTER 12 



if one of the two unions deeded tor recip- 

 rocal translocation occurs, so does the other. 

 Such is the case in the nucleus of the Dro- 

 sophila sperm just alter fertilization. In 



oocytes and probably in other cells that have 

 a relatively large nuclear volume, the dis- 

 tance between the broken ends of nonhomo- 

 logs is so great that reciprocal translocations 

 are comparatively rare and. even if one cross 

 union occurs, the two other broken ends 

 usually tail to join to each other, so that 

 onlj half of a reciprocal translocation — a 

 half-translocation — is produced. The loss 

 or behavior of the unjoined fragments usually 

 causes descendent cells to die or to be ab- 

 normal, as would be expected. Half-trans- 

 locations can also result when heterozygotes 

 for a eucentric reciprocal translocation un- 

 dergo segregation (see Figure 12-6), and 

 only one of the two reciprocals is present 

 in a gamete. 



Some children with Down's syndrome 

 have 46 chromosomes. These chromosomes 

 include — in addition to two normal num- 

 ber 21*s — an autosomal pair (from group 

 13-15 or from group 16-18) which is het- 

 eromorphic. one member being longer than 

 usual. The extra piece is probably the long 

 arm of 21, so that the individual is hyper- 

 ploid for 21, being almost trisomic 21. In 

 some cases the mother is phenotypically nor- 

 mal although she is heterozygous for a 

 eucentric reciprocal translocation between 

 21 and, for example, 15. Her chromosome 

 constitution can be represented by 15, 15.21 

 (centromere of 15), 21.15 (centromere of 

 21 ). 21. An egg containing 21 and 15.21 

 (the half-translocation) fertilized by a nor- 

 mal sperm (containing 21 and 15) produces 

 the almost-trisomic-21 mongoloid under dis- 

 cussion. (The break in 15 must have been 

 so close to the end that the hypoploid seg- 

 ment in the half-translocation mongoloid in- 

 dividual was not lethal.) In other cases 

 such half-translocational mongoloids have 

 half-translocational nonmongoloid mothers 



with 45 chromosomes. These mothers have 

 only one normal 21, one normal 15, for 

 example, and the half-translocation 15.21. 

 The hypoploidy for both 21 and 15 must 

 be small enough to be viable in the mother, 

 who can produce the aneuploid gamete that 

 makes her child mongoloid. (Note in the 

 cases cited above no relation exists between 

 mother's age and the occurrence of half- 

 translocational mongoloid children. ) 



In the second case in which two chromo- 

 somes are broken once, the chromosomes 

 are homologs (ABCDEFG.HIJ). The 

 breaks are usually at different places, for 

 example, between A and B in one chromo- 

 some, and between D and E in the other. 

 Here, also, reciprocal translocation can occur 

 two ways. The aneucentric type produces 

 a dicentric and an acentric chromosome 

 whose fate can be readily predicted. The 

 eucentric type produces two eucentric chro- 

 mosomes, the BCD region being deficient 

 in one and duplicated in the other. 



From the preceding discussion, one would 

 expect eucentric reciprocal translocations to 

 tend to be eliminated from the population 

 soon after arising by mutation, since they 

 are usually heterozygous and cause about 

 50% of gametes to be half-translocational 

 aneuploids. Certain eucentric reciprocal 

 translocations, however, seem to be excep- 

 tions. In these cases, almost a whole arm 

 of each chromosome is mutually exchanged. 

 Such whole-arm reciprocal translocations — 

 when heterozygous in Drosophila and prob- 

 ably in most other species — tend to synapse 

 and disjoin in the following way: at synapsis 

 the heterozygous reciprocal translocation 

 forms an X configuration composed of two 

 tetrads (Figure 12-6). Later, when homol- 

 ogous centromeres repel each other, alter- 

 nate centromeres move toward the same 

 pole, so that as the chiasmata move towards 

 the ends, a zigzag arrangement of four dyads 

 results (Figure 12-6). Because of this al- 

 ternate centromeric orientation, anaphase I 



