Procedure of Back Cross Method of Breeding in Self Pollinated Crops
The plan of back cross method depend upon whether the gene being transferred is recessive or dominant. The plan for transfer of a dominant gene is quite simple than for recessive gene.
Transfer of Dominant Gene:
Let us suppose that a high yielding and widely adopted variety ‘a’ is susceptible to stem rust (rr) and another variety ‘b’ is poor yielding but resistant to stem rust (RR) i.e dominant to susceptibility. In this back cross programme rust resistance trait is transfer from donor parent into a recurrent parent.
Variety ‘A’ is crossed with variety ‘B’ in which variety ‘A’ is used as female parent which is recurrent and variety ‘B’ is used as donor parent.
2) F1 Generation:
During the second year F1 plants are backcrossed to variety ‘A’ since all the F1 plants will be heterozygous for rust resistance. Selection for rust resistance is not necessary.
3) First Back Cross Generation:
In the third year half of the plant would be resistant and remaining half would be susceptible to stem rust, rust resistant plants are selected and backcross to variety ‘A’.
4) BC2 –BC6 Generation:
In each backcross generation, segregation would occur for rust resistance. Rust resistant plant are selected and backcrossed to the variety ‘A’ selection for plant type of variety ‘A’ may be practised particularly in BC2 and BC3 generation.
5) BC6 Generation:
On an average the plant will have 98490 genes from variety A rust resistant plants are selected and selfed, their seeds are harvested separately.
6) BC6 F2 Generation:
Individual plant progenies are grown from the selected plants. Rust resistance once plant, which are similar to variety ‘A’ are selected and selected plants are harvested separately.
7) BC5 F3 Generation:
Individual plant progenies are grown homozygous progenies resistant to rust and similar to plant type of variety ‘A’ harvested in bulk. Several similar progenies are mixed to constitute the new variety.
8) Yield Test:
The new variety is tested in R.Y.T i.e replicated yield trials along with the variety ‘A’ as a check. Plant type dates of flowering date of maturity, quality, etc are critically evaluated. The new variety would be identical to variety ‘A’ in performance. Therefore detail yield test are not required, and the variety may be directly released for cultivation.
Transfer of Recessive Gene:
When rust resistant is due to a recessive gene, all the backcross cannot make one after other. After the first backcross and after every two backcrosses F2 must be grown to identity the rust resistant plants. The F1 and the back cross progenies are not inoculated with rust because they would be susceptible to rust. Only F2 is tested for rust resistant.
The recurrent parent is crossed with rust resistant donor parent. The recurrent parent is generally used as female. i.e ( rr X RR).
2) F1 Generation:
F1 plants are backcrossed to the recurrent parent.
3) BC1 Generation:
If rust resistance is recessive all the plant will be rust susceptible. Therefore, there is no test for rust resistance. All the plants are self- pollinated.
4) BC1 (F2) Generation:
Rust resistance plants are selected and backcrossed with recurrent parent. i.e variety ‘A’. Selection is made for the plant type and other characteristics of the variety ‘A’.
5) BC2 Generation:
No rust resistance test, plants are selected, which is identical to the recurrent parent ( A) and backcrossed with the recurrent parent.
6) BC3 Generation:
No disease resistance test. The plants are self – pollinated to raise F2. selection is made for the plant type identical to variety ‘A’.
7) BC3 F2 Generation:
Plants are inoculated with stem rust. Rust resistant plant, similar to ‘A’ are selected and backcrossed to variety ‘A’.
8) BC4 Generation:
No rust resistance test plants are backcrossed to variety ‘A’.
9) BC5 Generation:
No rust resistance test plants are self pollinated to raise F2 generation.
10) BC5 (F2) Generation:
Plants are subjected to rust epidemic, resistance plant for rust and having similar characteristic of variety. ‘A’ is selected and self seed are harvested separately.
11) BC5 (F3):
Individual plant progenies are grown and subjected to rust epiphytotic selection is done for rust resistance and for characteristics of variety ‘A’ seeds from several similar rust resistant homozygous progenies are mixed to constitute new variety.
12) Yield Test:
Same as in case of transfer of dominant gene.