Translocation heterozygotes

When translocation is present in only one chromosome this will be a translocation heterozygote and when translocation is present in both non-homologous chromosome then this will be a translocation homozygote.
In translocation homozygote there is no problem during meiotic pairing but in case of Translocation heterozygotes, due to pairing between homologous segments of chromosomes, a cross shaped figure involving 4 chromosomes will be observed at pachytene sub-stage of meiosis I.
These 4 chromosomes at metaphase I can have one of the three orientations –

Alternate – In this orientation alternate chromosome (I and III) will be oriented toward the same pole.
Adjacent I – In this orientation adjacent chromosome (side by side) having non-homologous centromere will orient towards the same pole.
Adjacent II – In this orientation adjacent chromosome (I and III) having homologous centromeres will orient towards the same pole.

Irrespective of whether segregation during meiosis I is alternate or adjacent, 2 types of gametes will be produced.
Alternate segregation will produce balanced gametes but adjacent segregation will produced gametes which will be duplicated for some genes or deficient for few others, such unbalanced gametes will not be viable.
This is the reason why organisms with heterozygous translocation are semi sterile.
Such instance are very common in wheat, maize, pea, datura etc.In plants like Rhoeo and Oenothera, translocation heterozygote have become stable due to the presence of balanced lethals or lethals that are not expressed under heterozygous conditions.

Translocations have been found in human beings also. They can be detected by karyotype analysis.
Reciprocal translocation between chromosome 5 and 18 are well documented.
Sometimes reciprocal translocation results into heteromorphic autosomal pairing involving chromosomes 13, 14, 15, 16, 17 or 18 and chromosome 21 (the trisomy of which results into Down syndrome).
One of the autosomes of such a pair is normal whereas the other one has an extra piece, corresponding to the long arm of chromosome 21; in addition to the normal pair of chromosome 21.Such individuals are virtually trisomic for chromosome 21 and therefore show Down’s syndrome.
They arises of a female with reciprocal translocation between chromosome 15 and 21 marries a normal male.

Translocations results into changed linkage relationships between genes because non linkage groups are established.
Independently assorting genes become linked and linked genes began to show independent assortment provided their linkage groups are changed because of translocation.
Chromosomal translocation result into change in the sequence of gene but not their number, very frequently result into a change in phenotype of the organism.
This clearly shows that the expression of some genes is affected by their position on a chromosome. This effect is known as position effect.

Breeding behaviour of translocation heterozygotes

Presence of translocation heterozygosity can be detected by presence of semisterlity and low seed set.
This can then be confirmed at meiosis by quadrivalent (4 univalent) formation. As shown in figure only two types of functional gametes are formed which results from alternate orientation.
The functional gametes will give rise three kinds of progeny namely –

Normal,
Translocation heterozygote,
Translocation homozygote.

These three types would be obtained in 1:2:1 ratio.

Complex translocation heterozygotes

Complex translocation can be obtained by recurrent irradiation or by intercrossing simpler translocation stocks to yield complex one.
Both these methods were used successfully. In both cases where complex rings could be synthesized, high sterility was associated with the ring, which became a major barrier in utilization of these rings in Oenothera.
Genus Oenothera was studied 1920 – 1930 and cytogenetics structure leading to evolution in this genus was examined. In this genus 2n = 14 and all 7 chromosomes of a haploid complement have median centromere.
In O. Lamarkiana a ring of only 12 chromosomes instead of ring of 14 chromosomes is obsereved and 2 chromosomes forming a bivalent is seen.
In this genus Oenothera, few permanent and few temporary translocations are found which develops into different species like O. biennis, O.strigosa and O.irrigua, O.hookeri.
These species differ in phenotypes like flower size etc and can be identified.

Translocation Tester Sets

In many chromosomes of a particular species may lack any type of markers which could otherwise enable identification of individual chromosomes.
Genetic and environmental variations in chromosomal phenotype make it difficult for identification of individual chromosome.
Tester sets (Translocation homologous with known chromosomes involved in translocation) is used to identify such chromosomes, because individual chromosome involved in a structural change (Translocation) or in anenploidy can then be identified by making crosses to the tester set.
The significance of such a tester set can be understood in identification of unknown translocation, trisomics and monosomics.

For the synthesis of translocation tester set seeds were irradiated and grown M1 population.
Then the plants of M1 population selected were partial pollen sterility is found and meiotic behaviour of these selected plants were studied to confirm translocation heterozygotes (a ring of 4 chromosomes is seen).
Then selfing is done between such translocated heterozygotes, as self pollination crates translocation homozygosity, and harvest the seed of individual plants separately.
Then grow single plant progenies and select each progeny for plants with only bivalents (as bivalent showing translocation homozygote).
This generation is M2 generation. M2 generation plants were crossed with normal plants and study the meiotic behaviour of these crossing result or F1 hybrids. Identify chromosome involved in translocation. Such translocation tester sets are identified in maize, barley, pea, rye and tomato.

Robertsonia Translocation

Robertsonia translocation was first discovered by W.R.B Robertson in 1911.
It is also known as centric fusion translocation. Chromosomal breakage of acrocentric chromosomes near the centromeres results into robertsonia translocation.
This chromosomal breakage results into 2 segments, one large segment and another small segment.
The larger segments fuse together to form a new submetacentric or metacentric character.
While smaller segments may fuse to form new character or may be lost but in both case they are of no use as smaller part of acrocentric character contain many non essential heterochromatic DNA.
Person with balanced robertsonia translocation will have 45 chromosomes only.
The human chromosomes of D and G groups shows this translocation i.e. chromosome no. 13, 14, 15 and 21 and 22.

Eg: Although translocations generally have been of greater importance in the plant kingdom than in the animal kingdom, one type of translocation has had some evolutionary significance in animals.
This aberration, described as a centric fusion, is actually a translocation between two chromosomes.
It has been found in Drosophila, arthropods, certain birds, and mammals.
Studies with Drosophila have indicated that translocations, particularly centric fusions, have caused evolutionary changes of chromosome numbers in various species.
Their principal effect has been to reduce the chromosome numbers.
Example of centric fusions are given, which presents the possible evolutionary patterns in two subgenera, Drosophila and Sophophora.
Each fusion involves a translocation of almost the entire arm of a rod chromosome to produce a V-shaped chromosome.

Translocations of small portions of an arm are rare in Dorsophila.
There are other differences besides the translocations in the chromosomes of the species represented, including inversions, and the genetics of these organisms is rather complex, especially since hybridization is possible among many of the species.
A similar reduction in chromosome number is exhibited by the plant Crepis.
The more primitive species of Crepis have a basic chromosome number of six (n = 6), whereas the more advanced species have basic number of three, four, and five. Reciprocal translocations have contributed greatly to this evolutionary trend toward reduction in chromosome number.
In those cases where centric fusions lead to a decrease in chromosome number, little genetic material is lost from the genome, since most of the translocated chromosome is retained. Thus there is little loss in viability or fertility of the offspring of later generations.

B-A Translocation

In 1947, Roman reported a special type of translocation in maize involving translocation between two normal chromosomes A and B, in these chromosomes breakage and reunion takes place.
Chromosome A breaks at one point and same thing in B chromosome, a breakage takes place.
Both the broken pieces reunite as A with a piece of B chromosome and B with a piece of A chromosome.
This translocation takes place only in one chromosome i.e. it takes place in heterozygotic condition.
At metaphase plate a quadrivalent is found having A, AB, B and BA.