Principles of Plant Genetics and Breeding

Year 1
Year 2 F 1
Year 3
Year 4
Year 5
Year 6
Year 7
Year 8
Year 9
Year 10
Year 11
Year 12
Year 13
Year 8 Grow BC 5 F 1 plants to be selfed. Select
several hundreds (300–400) desirable plants
and harvest them individually.
Year 9 Grow BC 5 F 2 progeny rows. Identify and
select about 100 desirable non-segregating
progenies and bulk.
Year 10 Conduct yield tests of the backcross with the
recurrent cultivar to determine equivalence
before releasing.
Comments: dominant gene transfer
The steps for transferring a dominant gene are straightforward.
Following the first cross between the parents,
phenotypic selection is adequate for selecting plants
that exhibit the target trait. Recessive genotypes are discarded.
The recurrent parent traits are not selected at
BREEDING SELF-POLLINATED SPECIES 305
Generation
Action
A (susceptible) B (resistant)
P rr × RR
Cross parents to produce F1 BC 1F 1
BC 2F 1
BC 3F 1
BC 3F 2
BC 3F 3
BC 4F 1
BC 5F 1
BC 6F 1
BC 6F 2
BC 6F 3
Rr × rr
rr Rr × rr
rr Rr × rr
rr Rr
rr rr Rr RR
rr Rr RR RR
RR × rr
Rr × rr
rr Rr × rr
rr Rr
rr Rr RR
rr Rr RR RR
Backcross F 1 to parent A (rr)
Discard susceptible (rr) plants;
backcross Rr to rr
Discard susceptible plants;
backcross Rr to rr
Grow BC3F1 Discard susceptible plants; progeny row
Select BC 3F 3 progenies with resistance
and high yield
Backcross superior lines to rr
Discard susceptible plants;
backcross resistant plants to rr
Resistant cultivar
Figure 16.6 Generalized steps in breeding a dominant trait by the backcross method. The exact steps vary among
breeding programs.
this stage. The next cross is between the selected F 1 and
the recurrent parent. This step is repeated for several
cycles (BC n ). After satisfactory recovery of the recurrent
parent, the selected plant (BC n F 1 ) will be homozygous
for other alleles but heterozygous for the desired traits.
The last backcross is followed by selfing to stabilize the
desired gene in the homozygous state. All homozygous
(BC n F 2 ) recessive segregates are discarded.
Steps: recessive gene transfer
= discard
= desired
Years 1–2 These are the same as for dominant gene
transfer. The donor parent has the recessive
desirable gene (Figure 16.7).
Year 3 Grow BC 1 F 1 plants and self, harvest, and
bulk the BC 1 F 2 seed. In disease-resistance
breeding, all BC 1 s will be susceptible.