404 Intrachromosomal Aberrations 



In Drosophila, translocation homozygotes are largely lethal. 



How simple translocations and reciprocal translocations occur 

 is not definitely known. As the simple type is very rare, perhaps 

 does not exist, there is no need to speculate upon how it may 

 arise. Reciprocal translocations are well known both in wild 

 populations, where they have sometimes been of considerable 

 evolutionary significance, and in plants that have been sub- 

 jected to X-rays. One of the more likely explanations is that a 

 break and a realignment follow illegitimate crossing over be- 

 tween nonhomologous chromosomes. 



^leiotic configurations are very interesting in plants or ani- 

 mals that are heterozygous for a reciprocal translocation. They 

 are easy to figure out if we remember our rule that like parts of 

 a chromosome pair only with like parts. If an organism is 

 heterozygous for one reciprocal translocation, it has a set of 

 four chromosomes, no two members of which are alike. Let us 

 suppose that the original stock had one pair of chromosomes 

 designated abed and another slightly shorter pair k 1 7n (Fig. 

 109). 



If one reciprocal translocation had occurred, two new chromo- 

 somes would be present as would one of each of the original 

 pairs. The new chromosomes would be variously constituted, 

 depending upon where the translocated pieces had broken off, 

 but they might well he ab m and kl e d. Since the a pairs with 

 the a, and each other segment pairs at zygotene with the cor- 

 responding segment, a cross-shaped configuration would be pro- 

 duced. It would not necessarily be a regular cross; that would 

 depend upon the relative lengths of the original chromosomes 

 and of the translocated pieces. Unless one of the translocations 

 was very short, however, one or more chiasmata would form in 

 each arm, and they would be especially obvious at diplotene. 



The metaphase configuration would depend upon the number 

 and position of the chiasmata in each arm and also upon the 

 degree of terminalization of the chiasmata. If terminalization 

 was complete, and if at least one chiasma had formed in each 

 arm, the metaphase configuration would be a ring of four chro- 

 mosomes. Such rings have been found in a number of plants that 

 have a reciprocal translocation. If terminalization is not com- 

 plete, the interstitial chiasmata will be numerous and the 



