In this article we will discuss about the interchanges involving more than two pairs of chromosomes.
When 3 or more pairs of non-homologous chromosomes are involved in translocation, it is called a complex translocation, complex interchange or multiple translocation. In a single cell, two or more independent heterozygous interchanges show two or more quadruples at MI.
Such interchanges are obtained by crossing two translocation lines where chromosomes involved in the two translocations are different. When two interchanges have one chromosome in common, i.e., involve 3 pairs of chromosomes, a ring or chain of six chromosomes is produced.
In such a case, there are two conditions:
(i) The chromosome common to both interchanges does not have a normal homologue since one of the homologous is involved in translocation with one non-homologous chromosome, while the other homologue has the trans-located segment from the other non-homologous chromosome.
This type of interchange complex is obtained by crossing two homozygous interchange lines that have one common chromosome involved in the translocations. (Fig. 14.12a).
(ii) One homologue of the common chromosome is involved in both the interchanges, while its homologue is normal; the interchanges are located either in the same arm or in the separate arms of this chromosome (Fig. 14.12b). This type of interchange originates from crossing over in the differential segment of the first type of interchange described above (i.e., the two homologues of the common chromosome are involved in different translocations; Fig. 14.12 a).
Ten different segment can be distinguished in the pachytene configurations of translocation heterozygotes involving three chromosomes : two non-interchanged segments A and F; four interchanged segment B, C, D and E; two interstitial segments G and H and two differential segments I and J (Fig. 14.12a, b).
The regions between the central centromeres (centromeres of the central chromosomes) and the two interchange points are called differential segments. Chiasmata may or may not be formed in each of these 10 segments; thus there are 210 (=1024) different possible MI configurations.
However, many of these configurations have the same shape (isomorphic), and the most frequent MI configuration is a chain of 6 chromosomes.
Methods of synthesizing complex interchanges have been suggested by Burnham in 1962. In 1965, Sisodia and Shebeski synthesized an interchange stock in barley in which all the 7 chromosome paris were involved, and a ring of 14 chromosomes was produced. But this plant was completely sterile.
Complex translocations have been produced by several workers in different plant species, such as, barley, Triticum monococcum, Campanula, Datura, maize, rye and several other organisms. However, such interchanges are completely sterile because there is no mechanisms to promote alternate segregation in the complex interchange multiples.