Principles of Plant Genetics and Breeding

306 CHAPTER 16
Year 1
Generation
Action
A (susceptible) B (resistant)
P RR × rr
Cross parents to produce F1 Year 2 F 1/BC 1
Year 3
Year 4
Year 5
Year 6
Year 7
Year 8
Year 9
Year 10
Year 11
BC 1F 2
BC 2
BC 3
BC 3F 2
BC 4
BC 4F 2
Rr × RR
Self
RR × rr Rr RR
RR × Rr
Rr RR
Self
RR × rr Rr RR
Figure 16.7 Generalized steps in breeding a recessive trait by the backcross method. The exact steps vary among breeding
programs.
Year 4 Grow BC 1 F 2 plants and screen for desirable
plants. Backcross 10–20 plants to the
recurrent parent to obtain BC 2 F 2 seed.
Year 5 Grow BC 2 plants. Select 10–20 plants
that resemble the recurrent parent and
cross with the recurrent parent.
Year 6 Grow BC 3 plants, harvest and bulk the
BC 3 F 2 seed.
Year 7 Grow BC 3 F 2 plants, screen, and select
the desirable plants. Backcross 10–20
plants with the recurrent parent.
Year 8 Grow BC 4 plants, harvest, and bulk the
BC 4 F 2 seed.
Year 9 Grow BC 4 F 2 plants, screen, and select
the desirable plants. Backcross 10–20
plants with the recurrent parent.
Year 10 Grow BC 5 plants, harvest, and bulk the
BC 5 F 2 seed.
Year 11 Grow BC 5 F 2 plants, screen, and backcross.
Year 12 Grow BC 6 plants, harvest, and bulk the
BC 6 F 2 seed.
Rr
Self
RR Rr rr
Backcross to parent A (RR);
self
Discard susceptible plants;
backcross resistant plants to RR
Backcross BC2 plants to RR
Grow BC 3 plants and self
Discard susceptible plants; backcross
resistant plants (BC3F2) to RR
Grow BC4 and self
Discard susceptible plants;
bulk seed from resistant plants
= discard
= desired
Year 13 Grow BC 6 F 2 plants and screen; select
400–500 plants and harvest separately
for growing progeny rows.
Year 14 Grow progenies of selected plants, screen,
and select about 100–200 uniform progenies;
harvest and bulk the seed.
Years 15–16 Follow the procedure as in breeding for a
dominant gene.
The key difference between the transfer of dominant
and recessive alleles is that in the latter case, phenotypic
identification is not possible after a cross. Each cross
needs to be followed by selfing so that the progeny with
the homozygous recessive genotype can be identified
and backcrossed to the recurrent parent.
Comments: recessive gene transfer
1 Backcrossing does not have to be conducted in the
environment in which the recurrent parent is adapted
because all that is needed is to be able to identify and
select the target trait.