The following points highlight the top fifteen uses of translocation in genetics. The uses are: 1. Study of chromosome behaviour during meiosis 2. Assignment of centromere position 3. Interchanges as genetic markers 4. Determination of unknown locus of a gene and chromosome mapping 5. Association of linkage groups or genes to specific chromosomes and a Few Others.
Uses of Translocation
- Study of chromosome behaviour during meiosis
- Assignment of centromere position
- Interchanges as genetic markers
- Determination of unknown locus of a gene and chromosome mapping
- Association of linkage groups or genes to specific chromosomes
- Testing of the independence of linkage groups
- Association of different linkage groups
- Determination of the initiation of chromosome pairing
- Production of duplication
- Evolution of allopolyploids
- Gamete selection
- Enlarging the genome
- Evolution of karyotype
- Interchanges as a source of trisomies
- Interchanges may be used for pest control
Translocation: Use # 1. Study of chromosome behaviour during meiosis:
Interchanges may be used to study chromosome behaviour during meiosis, such as, various configurations of chromosomes due to homologous pairing, orientation and segregation in reciprocal translocations and in complex interchanges.
Translocation: Use # 2. Assignment of centromere position:
Pattern of reduction in recombination values in the interstitial region is used to determine the relative position of breakpoints, marker genes and the centromere.
Translocation: Use # 3. Interchanges as genetic markers:
Interchange breakpoints can be plotted on linkage maps in the form of partial sterility loci (PS). Homozygous interchanges have normal fertility but when they are crossed to normal plants, the individuals show partial sterility.
In F2, fertility and sterility segregate in 1 : 1 ration, while the marker genes segregate in 3 : 1 ratio. The joint segregation of PS and the marker genes leads to the 3 : 3 : 1 : 1 ratio in case of independent assortment. In cases of the failure of independent segregation, recombination values between the PS and genetic markers are estimated.
Translocation: Use # 4. Determination of unknown locus of a gene and chromosome mapping:
Interchanges with known breakpoints may be used to determine the unknown locus of a gene. The homozygous interchange lines are crossed with the plant carrying the mutant gene. Recombination values between PS and the genes are estimated using F2 or F2 and F3 data or from the back cross data. Genes can be mapped on chromosomes using the recombination values.
Translocation: Use # 5. Association of linkage groups or genes to specific chromosomes:
In several crops, e.g., barley, maize and peas, translocations have been used for assigning specific linkage groups or genes to specific chromosomes. The procedure involves linkage study using F2, F2 + F3 or test cross data and cytological study. For example, in barley, chromosomes 1 to 7 are now designated as the seven linkage groups and genetic markers have been assigned to them (Table 14 4).
Translocation: Use # 6. Testing of the independence of linkage groups:
Interchanges may be used to test the independence of linkage groups. Using this approach, Kramer and associates showed in 1954 that the then linkage groups III and VII of barley were not independent, but were carried in the same chromosome (Table 14.4).
Translocation: Use # 7. Association of different linkage groups:
Due to interchange, genes of different linkage groups become associated together in a single chromosome. Thus new linkage groups may be created using translocations.
Translocation: Use # 8. Determination of the initiation of chromosome pairing:
Intercrossing between interchanges involving the same chromosomes provide information about the points at which chromosome pairing is initiated. The S of the intercrosses involving the same chromosomes show a number of configurations depending upon the breakpoint located in different arms of the chromosomes involved.
The most critical evidence comes from intercrossing the interchange lines in which the breakpoints are located in separate arms of both the chromosomes involved in the interchange. From such crosses, only bivalents are expected if pairing is always initiated at or near the ends of chromosomes, whereas only quadrivalents are expected if pairing is initiated only at or near the centromeres (Fig. 14.17).
If pairing is initiated at both the positions, i.e., at the centromeres as well as the ends, a distinct quadrivalent configuration is formed. On the basis of these assumptions, it was demonstrated that pairing is initiated near the ends of chromosomes.
Working with maize interchanges, Burnham and coworkers in 1972 concluded that the initial pairing occurs in one or two short segments adjacent to the end segment and there are multiple secondary sites which vary in time or in order of pairing.
Initiation of chromosome pairing at the ends was further confirmed by the study of 8- chromosome barley haploids obtained by crossing the 8-paired (2n =16) barley line of H. vulgare with H. bulbosum by Kasha et al. in 1975.
The 8-paired barley line was produced in the progeny of a tertiary trisomic from the interchange T5-7a; it had deletion of the end segments of the arms of the extra chromosome (trans-located chromosome). Thus the extra short chromosome had homologous regions proximal to the centromeres of two non-homologous chromosomes (involved in translocation).
The meiosis in the 8-chromosome haploid showed 8 univalents, because the initiation sites for pairing near the ends were deleted.
Translocation: Use # 9. Production of duplication:
Interchanges have been used to produce duplications. Duplications are obtained in the F2 generation when two interchanges involving the same chromosomes but with different break-positions are intercrossed.
There are two types of interchanges which may give such progeny:
(i) Interchanges where breaks are located in the same arm but at different positions in both the chromosomes involved in the interchange. In this case duplication would be obtained for the segments differentiating the two parental interchanges, and it would be obtained for both the chromosomes.
(ii) Both the interchanges have similar breakpoints for one chromosome but different breakpoints for the other chromosome. In such a condition, the duplication would be obtained for the segment between the two breaks only in the chromosome that has different breakpoints (Fig. 14.18).
By crossing translocation T6-7a with T6-7d, Hagberg for the first time in 1962 produced duplication for a large segment of the short arm of chromosome 6 in barley. Such duplications may decrease the dosage of certain genes and thereby increase the disease or insect pest resistance or enzymatic activity of the genes which may be located in the duplicated segment.
Duplication in barley produced by Hagberg increased the activity of the enzyme alpha-amylase.
Translocation: Use # 10. Evolution of allopolyploids:
Interchanges may serve as an isolating mechanism for the evolution of allopolyploids. If two lines belonging to a single species, each containing a series of different translocations are crossed and the chromosomes of the F, are doubled, the resulting plant may behave as an allopolyploid instead of an autopolyploid.
Translocation: Use # 11. Gamete selection:
In 1946, Burnham proposed the “Oenothera method” of gamete selection for the isolation of inbred lines with the help of multiple interchanges. The first step in this method is the synthesis of interchanges involving all the chromosomes in one complex. The second step consists of crossing this multiple interchange homozygote to the heterozygous line from which an inbred is to be derived.
Each F, individual from this cross would have a different combination of genes from the heterozygous line combined with the multiple interchange gamete. The F, plants are selfed separately to obtain separate F2 populations from them.
Plants having the standard karyotype and MI configuration in each of the F2 population would be an inbred line; such an inbred line will have the same combination of genes from the heterozygous line that was present in the concerned Fx plant.
However, with an increase in number of chromosomes involved, sterility increases in the interchange heterozygotes. In many crops, complex interchanges show 100% sterility. The production of inbred lines through the Oenothera method requires the formation of fertile spores by the complex interchange heterozygote which is possible only through alternate segregation in the interchange multiples.
This requires some sort of genetic regulation, including the inclusion of balanced lethals.
Translocation: Use # 12. Enlarging the genome:
Interchanges may be used to increase the chromosome number of a species. A barley (2n =14) line with 8 pairs of chromosomes (2n = 16) was reported by Wiebe in 1975; this line was obtained in the progeny of a tertiary trisomic produced by crossing the variety Betzes with the interchange line T5-7a.
The extra (trans-located) chromosome of the tertiary trisomic became shorter, approximately 1/3 of its normal length, due to a deletion of both its terminal ends. This short chromosome was occasionally transmitted through pollen.
When the pollen carrying the extra chromosome fertilized an egg containing the same extra chromosome, a true breeding 16-chromosome plant would be produced. The extra chromosome formed a separate bivalent. The genes present in the extra chromosome are duplication of those present in the normal genome; in effect, the 8-pair barley line is tetrasomic for these genes. Therefore, the extra chromosome would have similar value as duplications.
Translocation: Use # 13. Evolution of karyotype:
Changes in the karyotype resulting from translocations may be of two types; changes in chromosomes structure and change in chromosome number.
(i) Change in chromosome structure:
Interchanges may alter the centromere position (arm ratio) if unequal segments are exchanged (Fig. 14.1, 14.2.) The size of the chromosome may also be changed from small to large.
(ii) Reduction in the genome size:
Centric fusion (Robertsonian translocation) involving two acrocentric chromosomes produces a large metacentric chromosome and a very small chromosome containing a centromere with or without minute arms (Fig. 14.2). The small chromosome is generally devoid of genes and may be eliminated from the cell resulting in a reduction in the number of chromosomes in the genome.
Translocation: Use # 14. Interchanges as source of trisomies:
Interchanges are good source of various kinds of trisomies. Trisomies are produced when 3 : 1 disjunction occurs in a ring/chain of 4 chromosomes resulting in the formation of gametes containing n + 1 chromosomes (Fig. 14.6).
If both normal and one trans-located chromosomes go to one pole, the trisomic will be a tertiary trisomic. But if both the trans-located chromosomes and one normal chromosome move to the same pole, the trisomic will be an interchange trisomic.
Translocation: Use # 15. Interchanges may be used for pest control:
Males heterozygous for multiple interchanges will produce nonfunctional sperms, therefore, such males may be released in large numbers for insect control. Translocations may be sued for other purposes, such as, to study the somatic pairing in Diptera. They may also be used to transfer desired character into an inbred.