The following points highlight the five specific types of translocation in genetics. The types are: 1. Pseudo-Iso-Chromosomes 2. Interdependent Rings 3. Cyclic or Progressive Translocation 4. Duplication-Deficiency Interchange Heterozygotes 5. Interchanges between Normal and Supernumerary Chromosomes.
Translocation: Type # 1. Pseudo-Iso-Chromosomes:
This term was coined by Caldecott and Smith in 1952: it may be defined as a chromosome the two ends of which are homologous but the segments proximal to the centromere are non-homologous (Fig. 14.13). Pseudo-iso-chromosomes are produced as a result of reciprocal translocation of the end segment of short arm of one chromosome and that of the long arm of its homologue.
This produces two pseudo-iso-chromosomes that have duplication-deficiency in such a way that they complement each other by the two arms. Such chromosomes pair near the ends at meiosis similar to iso-chromosomes but their segments proximal to the centromeres (interstitial region) remain unpaired due to non-homology (Fig. 14.13). They have been reported in maize and oats.
Translocation: Type # 2. Interdependent Ml Rings:
Interdependent rings have been reported in Campanula and Oenothera. Two interdependent rings of four chromosomes can result when four non-homologous chromosome are involved in reciprocal translocations of the terminal segments of both their arms in such a manner that the terminal segments in the two arms of each chromosomes are derived from the two non-homologous chromosomes, as shown in the Fig. 14.14.
During meiosis in the translocation heterozygotes, the four interchanged chromosomes and their normal homologues pair to produce two rings of 4 chromosomes each. The two rings are interdependent as the homologue of one differential segment (mid segment) in one ring is present in the other ring.
The following are only two combinations of the chromosomes from the two interdependent rings which will form viable spores:
(1) The trans-located chromosomes in one group, and
(2) Their four normal homologues in the other group.
Translocation: Type # 3. Cyclical or Progressive Translocation:
In such type of translocations, several breaks occur simultaneously and there are successive translocations and substitutions of end segments. Such translocations have been reported in Datura, maize and Drosophila.
Translocation: Type # 4. Duplication-Deficiency Interchange Heterozygotes:
When one of the trans-located chromosome segments is relatively smaller than the other, the female gamete containing one normal and one trans-located chromosomes produced by the interchange heterozygote may be viable, i.e., the small deficiency and larger duplication may be tolerated by the female gamete.
After fertilization by the pollen containing normal chromosomes, “deficiency-duplication interchange heterozygote” is produced (Fig. 14.15). Such plants may show pseudo-dominance for the gene located in the deficient segment. This type of interchange has been reported in T. inonococcum, maize, Drosophila and some other organisms.
Translocation: Type # 5. Interchanges between Normal and Supernumerary Chromosomes:
Interchanges may occur between normal (A-chromosomes) and B-chromosomes, e.g., in maize where they have been used for locating genes in chromosome arms. If the B-chromosome carrying the trans-located segment from an A-chromosome is eliminated from the cell, deficiency for the trans-located segment would be produced.
The concerned B-chromosome may undergo nondisjunction during pollen mitosis; as a result, the A-chromosome segment trans-located on to this B-chromosome would also become duplicated due to the nondisjunction.