Translocations 



403 



tion is a simple translocation. They 

 are probably rare, and some geneti- 

 cists have questioned their existence. 

 Far more common is a reciprocal 

 translocation, by which a piece of 

 one chromosome becomes exchanged 

 for a segment of a nonhomologous 

 chromosome. The exchanged seg- 

 ments need not be of equal length. 

 If not, they are often detected cyto- 

 logically by the differences in chro- 

 mosome size in the original and in- 

 terchanged strains (Fig. 108) . 



Like inversions, translocations may 

 be both homozygous and heterozy- 

 gous. Also like inversions, trans- 

 location homozygotes are little dif- 

 ferent cytologically from the normal 

 type. Since the two members of 

 each pair of chromosomes are alike, 

 a translocation homozygote will show 

 nothing but normally paired chro- 

 mosomes. The only difference ob- 

 servable cytologically between this 

 and the original type which gave 

 rise to the reciprocal translocation 

 may appear if the two translocated 

 pieces are of different size. If a long 

 segment was translocated from one 

 chromosome to another and a short 

 segment from the second to the first, 

 the chromosomes would be of differ- 

 ent lengths from those observed in 

 the original type. As is also true of 

 inversions, the presence of a homozy- 

 gous translocation may not be de- 

 tectable by cytological examination. 

 It will, however, bring about a change 

 in the linkage groups and therefore 

 can be detected by a genetic study. 



Fig. 108. Translocation in- 

 duced by X-rays in the grouse 

 locust, Apotettix eurycephalus 

 Hancock. Top, primary sper- 

 matocyte of a normal indi- 

 vidual showing six pairs of 

 chromosomes and an unpaired 

 X chromosome. Middle, pri- 

 mary spermatocyte of X- 

 rayed individual showing that 

 a piece of one of the homo- 

 logues of chromosome 1 had 

 broken off and become trans- 

 located to one homologue of 

 chromosome 4. Bottom, sper- 

 magonium of the X-rayed 

 individual. Note that in the 

 two lower figures one chro- 

 mosome 1 is shorter and one 

 chromosome 4 is longer than 

 normal. (Redrawn from Na- 

 bours and Robertson in Pro- 

 ceedings of the National Acad- 

 emy of Sciences.) 



