In this article we will discuss about:- 1. Origin of Translocation 2. Types of Translocation 3. Effects.
Origin of Translocation:
Translocations can originate in the following different ways:
(i) Translocations may originate spontaneously.
(ii) They may be induced by mutagens, viz., ionizing radiations and many chemical mutagens, since they induce chromosome breakage.
(iii) Translocations may be induced by growing plants in calcium-deficient media, as reported by Nilan and Phillips in 1957.
(iv) Translocations may be induced by oxygen applied at a high atmospheric pressure, as reported by Kronstad et al., in 1959 and Moutschen-Dahmen et al., in 1959.
(v) Translocations can be recovered from certain interspecific crosses since the concerned species differ for chromosomal rearrangements, including translocation, which become observable in their interspecific hybrids.
(vi) Genetically controlled breakage in the chromosomes may also produce translocations, such as, sticky gene (st) and DS-AC system in maize.
In 1914, Belling reported 50% pollen abortion and 50% seed set in crosses of Florida velvet bean which he termed as semi-sterility. Later in 1924, Belling and Blakeslee, working with Datum stramonium, concluded that non-homologous chromosomes could exchange segments.
The breeding behaviour of semi-sterility in Stizolobium deeringianum was explained in 1925 by Belling on the basis of “segmental interchange between non-homologues”.
In maize plant, semi-sterility was reported by Brink in 1927. In 1930, Burnham reported a ring of 4 chromosomes in the semi-sterile plant of maize. In the same year, McClintock showed that translocation heterozygotes produced a “cross-shaped configuration” at pachytene. In Drosophila, the first translocation where a piece of X chromosome was attached to the Y chromosome was reported by Stern in 1926.
Certain genes have been reported to induce chromosome breaks leading to the production of translocations. Genetically controlled systems of chromosome breakage have been observed in some cases. In maize, chromosome breaks occurred at AI of meiosis due to stickiness of chromosomes aberrations.
The DS-AC system in maize first described by McClintock in 1950 also causes structural changes by inducing chromosome breaks.
Interlocking of bivalents which takes place in certain species, such as, Tradescantia, also causes chromosome breakage leading to various aberrations. Translocations have been induced through various physical and chemical mutagens in several plant and animal species.
Translocations originate through chromosome breakage and reunion. It can also be interpreted on the basis of exchange model. The unit of translocation may be a chromosome (chromosomal translocation) or a chromatid (chromatid translocation).
Types of Translocation:
Translocation may be classified on the basis of the trans-located segment being present in the same, homologous or non-homologous chromosome, and the number of breaks involved in the translocation.
A. Classification on the basis of involvement of the same or different chromosomes:
1. Intra-chromosomal (internal) translocation or shift:
A segment of a chromosome is shifted from its original position to some other position within the same chromosome. It is of two types:
(a) Intra-Radial:
The shift occurs in the same arm (Fig. 14.1).
(b) Extra-Radial:
The shift occurs from one arm to the other arm (Fig. 14.1)
2. Inter-Chromosomal translocation:
A chromosomal segment is transferred from one chromosome to another one. It may be either fraternal or external.
(a) Fraternal:
The chromosome segment is trans-located into the homologous (Fig. 14.1).
(b) External:
The chromosome segment is trans-located into a non-homologous chromosome (Fig. 14.2).
The inter-chromosomal translocation may be divided into the following three groups.
I. Transposition:
Transfer of a chromosome segment from one chromosome to another chromosome is called transposition. It may be of the following types.
(i) Intercalation or insertion or insertional translocation:
The transposition occurs in an intercalary position.
(ii) Terminal transposition:
The segment is attached to the chromosomal end. However, terminal translocation is not possible so long as the telomere of the chromosome remains intact. Therefore, terminal translocation can occur only when the chromosome end is deleted or trans-located.
II. Reciprocal translocation or interchange:
Exchange of segments between two or more non-homologous chromosomes is called reciprocal translocation or interchange. It is of two types: asymmetrical or aneucentric and symmetrical or eucentric.
(i) Asymmetrical or aneucentric translocation:
After breakage, the broken acentric segments fuse to form a trans-located acentric chromosome, while the two chromosomes with centromeres fuse to produce a trans-located chromosome with two centromeres (dicentric). The dicentric chromosome will produce bridge at anaphase if the two centromeres move to opposite poles (Fig. 14.2).
(ii) Symmetrical or eucentric translocation:
Broken segments are exchanged between the two non-homologous chromosomes so that both the chromosomes involved in translocation possess only one centromere each (mono-centric) (Fig. 14.2).
III. Whole-Arm translocations or whole-arm transfers:
These are the special types of translocations where almost the entire chromosome arms are transposed or interchanged.
Such translocations are of three types:
(i) Centric fusion or Robertsonian translocation:
The long arms of two acrocentric chromosomes may fuse due to translocation to produce a metacentric chromosome, while their short arms fuse to form a very small chromosome (Fig. 14.2).
(ii) Dissociation:
Two metacentric chromosomes, one with long arms and other with short arms may produce two acrocentric chromosomes through translocation (Fig. 14.2).
(iii) Tandem fusion:
Such type of interchange is produced when the break in one chromosome occurs near the centromere and in the other chromosome, it occurs near the end. The result of such breakage and reunion may be a large acrocentric chromosome and a small metacentric chromosome, if both the chromosomes were originally acrocentric.
If one chromosome is a metacentric, the result o the interchange will be two acrocentric chromosomes, one being small and the other being large (Fig. 14.2).
B. Classification on the basis of the number of breaks involved:
According to this system Schulz-Schaeffer in 1980 divided the translocations into four classes:
(1) Simple (one break),
(2) Reciprocal (two breaks),
(3) Shift (three breaks), and
(4) Complex (more than three breaks) translocations.
1. Simple translocation:
In such a translocation, a segment of a chromosome becomes attached to the end of a non-homologous chromosome. In 1929, Painter and Muller reported such type of translocations in Drosophila. In view of the stability of telomere, intact chromosomal end cannot fuse with a chromosomal segment.
Therefore, cases of simple translocations are either reciprocal translocation in which a very small telomeric segment of one chromosome (apparently devoid of a detectable gene) is involved in a reciprocal translocation, or the telomeric region of the concerned chromosome gets deleted during the translocation.
2. Reciprocal translocation or Interchange:
In this type of translocation, segments are exchanged between two non-homologous chromosomes, therefore, it involves one break in each of the involved chromosomes (Fig. 14.2). Most of the translocations are reciprocal translocations. Such translocations have been extensively studied in various plant and animal species.
3. Shift type of translocation or Transposition:
It involves three breaks, and the broken segment is shifted (transposed) in the intercalary position (Fig. 14.1).
According to whether same or different chromosomes involved, shift is of two types:
(a) Intra-chromosomal shift:
Shift is confined to the same chromosome; the broken segment gets inserted either (i) within the same arm, or (ii) in the other arm of the chromosome.
(b) Inter-chromosomal shift:
A broken piece of a chromosome is inserted into an intercalary position of a non-homologous chromosome (Fig. 14.2).
4. Complex Translocations:
In such translocations, more than three breaks are involved. Mostly, such translocations are naturally occurring.
Phenotypic Effects of Translocation:
In general, no phenotypic effects of translocations are visible. But in case there is damage to the DNA during translocation, recessive mutations may arise. Translocations may also act as recessive lethals. Position effect may be produced by some translocations in certain organisms, such as, Drosophila, Oenothera etc.