Backcross Method of Breeding
Backcross Method of Breeding
A backcross may be defined as crossing of F1 hybrid with any of its parents. Backcrossing is a regular step in backcross method of plant breeding. Backcross method is particularly utilized to improve upon a variety which is otherwise good in all of its attributes except one or very few characters. Such few undesirable characters of this variety are improved by crossing this variety with a donor variety followed by repeated backcrossing. For this purpose two parents are chosen (i) recurrent parent, which belongs to the variety to be improved and (ii) a donor parent, which carries the desired character (s) to be incorporated in the recurrent parent. Recurrent parent and donor parent are crossed together and their F, hybrid is backcrossed to the recurrent parent. This backcrossing programme is continued for 6-8 generations.
Procedure of backcross method
Before deciding to use the backcross method of breeding, the following three conditions should be fulfilled-(i) Suitable recurrent and donor parents must be available. (ii) The expression of the character governed by the gene under transfer must be reasonably high in different backcross generations. Permitting selection of desirable plants in segregating backcross populations. (iii) Number of backcrosses should be large enough to recover the genotype of recurrent parent.
The procedures used for transfer of a dominant gene or a recessive gene through backcross method differ and therefore will be described separately. The two methods differ, because, while for transfer of a recessive gene, the backcross progeny will not show any phenotypic segregation (RR does not differ from Ra), thus permitting no selection of homozygous plants but for transfer of a dominant gene, the backcross progeny will allow selection of homozygous plants (rr will differ from Rr).
Transfer of dominant gene-
Following different steps are involved (a) Hybridization-Recurrent parent (usually called A) is crossed with donor parent (usually called B). Generally recurrent parent should be used as female so that if any selfing accompanies (unknowingly) crossing, selfed plants can be distinguished from F1 plants, because selfed plants being homozygous (rr) will show recessive character and F, plants being heterozygous (Rr) will show dominant character. If donor is used as female parent, plants raised from selfed seed (RR) and F1 plants (Rr) will be phenotypically similar for this character.
- F1 generation- All the F1 plants are backcrossed to the recurrent parent.
- First backcross generation (BC₁)- Plants having the gene under transfer, are again backcrossed to recurrent parent.
- Second to fifth backcrossed generations (BC2-BC5)- Plants, which express the transferred character and resemble the recurrent parent are again selected and backcrossed to recurrent parent. In this manner back crossing is done upto BC, or BC, generations, so that about 98.4% genotype of recurrent parent is restored alongwith the transferred gene.
- Sixth backcross generation (BC6)- In this generation plants with transferred gene (-98.4% recurrent parent) are allowed to self-pollinate and are harvested individually for obtaining this gene in the homozygous condition.
- Selfed progeny derived from BC6 (BC6 F2 generation)- Separate progeny rows of each selfed plant of previous generation (BC6) are grown and harvested individually.
- Selfed progeny derived from BC6 F₂ (BC6 F3 generation)- Again separate plant progeny rows are grown and seed of non-segregating progeny rows for the desirable character under transfer is bulked.
- Yield tests- Generally, extensive tests are not done but newly developed variety is compared with recurrent parent variety to verify that it resembles the recurrent variety A for all agronomic and other characters except the character that is transferred from donor variety B.
Transfer of a recessive gene-
In case of recessive gene transfer, a slightly different procedure is followed.
The recurrent parent (carrying the dominant allele, RR) and the donor parent (carrying the recessive allele, rr) are crossed together and their F₁ (heterozygous, Rr) is backcrossed with recurrent parent. This (BC,) will exhibit 1:1 segregation of homozygous (RR) and heterozygous (Rr) individuals, which cannot be distinguished phenotypically. Therefore, all these plants are allowed to self-pollinate (not immediately backcrossed as in case the transfer of a dominant gene- see above) and the resulting BC1F2 generation is grown. From BC1 F2 generation, plants showing resemblance to recurrent parent and carrying the character under transfer are backcrossed to recurrent parent to give BC2 generation. All these BC2 plants are again backcrossed to recurrent
parent to produce BC3 generation. The BC3 plants are self-pollinated and from the resulting BC3 F2 generation, plants showing resemblance to recurrent parent and also having transferred character are again backcrossed for two generations to produce BC4 and BC5 generations. BC5 plants are then self-pollinated to produce BC5 F2 (note that in this method, after every two back crosses selfing is required to isolate homozygous recessive plants). BC5F2 plants carrying the character under transfer are again allowed to self-pollinate and then allowed to grow in individual plant progenies. Progenies showing resemblance with recurrent parent are bulked. Such bulked population which is actually the potential new variety is tested in a manner similar to that used for transfer of a dominant gene.
Merits of backcross method
- The genotype of well adapted variety is recovered, which carries the desirable character transferred from the donor parent. Therefore, outcome or gain of the programme is already known.
- Extensive yield trials are not necessary which saves time.
- Role of environment is almost negligible, so that the off-season and green-house growing facilities may be utilized for developing the variety within a very short period.
- Breeder does not require handling large populations.
- This is the most commonly used method for intraspecific transfer of one or a few genes and for development of isogenic lines.
Demerits of backcross method
- The new variety will have a limited superiority over the already available variety i.e. only for the transferred character.
- Backcrossing in every generation will require lot of hybridization work amounting to lot of time and money.
- Interaction of Genes- Complimentary, Epistatic, Duplicate Gene etc.
- Inheritance of Multiple Factors
- Linkage- Types, Linkage Groups, Importance
- Concept of Crossing Over- Types, Mechanism, Significance
- Mutation- History, Range, Types, Stages etc.
- Aneuploidy- Monosomy, Nullisomy, Trisomy, Tetrasomy
- Procedure of Hybridization in Plant Breeding
- Selection Method of Plant Breeding
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