Backcross breeding is a method used to transfer one or few desirable traits from a donor parent to a recurrent parent with otherwise good qualities. It involves crossing a hybrid plant with one of its parents and selecting progeny that resemble the recurrent parent for further backcrossing. This helps recover most of the recurrent parent's genome while introducing the desired trait. Marker-assisted backcrossing can improve efficiency by selecting against donor genome regions outside the target locus and choosing rare recombinants near the target gene. The objective is to develop an improved variety like the recurrent parent but with the trait from the donor parent.

1. Dr. NAVEENKUMAR K.L
Assistant Professor
Dept. Of Genetics and Plant Breeding
UAHS Shivamogga
Backcross Breeding

2. What is Backcross ?
 A cross between a hybrid (F1 or a segregating generation) and one of its
parents is known as backcross.
TT X tt
(Tall) (Dwarf)
F1 Tt X TT OR Tt X tt
 In BACKCROSS METHOD, the hybrid and the progenies in the
subsequent generation are repeatedly backcrossed to one of the
parents of the F1 .

3. Why backcross method is used?
1. To transfer a major gene
2. In disease/pest resistance breeding
3. To transfer alien cytoplasm or to transfer cytoplasmic
male sterility
4. To transfer a transgene from already developed
transgenic line

4. objective of backcross method
 The objective of backcross method is to improve one or two
specific defect of a high yielding variety, which is well adapted
to the area and has a other desirable characteristics.

5. Requirement for a Backcross Programme
1. A suitable recurrent parent, which lacks in one or two
characters.
2. A suitable Donor parent that has an intense form of character
which is deficient in recurrent parent.
3. The character to be transferred must have high heritability and
governed by one or few genes.
4. Sufficient number of backcrosses.

6. History of Backcrossing
 Harlan and Pope, 1922
 Smooth vs. rough awn barley
 Wanted the smooth awns from European barleys in
the domestic barleys
 Decided to backcross smooth awn
 After 1 BC, progeny resembled Manchuria and they
were able to recover high yielding smooth awn types

7. Terminology
 Recurrent parent (RP) - parent you are transferring trait
OR
In backcross breeding, the parent to which few or more genes are
transferred from the donor parent is called the Recurrent parent.
 Donor or non-recurrent parent (DP) - source of desirable trait
OR
In backcross breeding, the parent from which few or more genes
are transferred to the recurrent parent is called the Donor parent.
 Progeny test - when trait is recessive

8. Procedure: Transfer of dominant gene through backcrossing
Rust resistance
Rust susptable

9. Procedure: Transfer of recessive gene through backcrossing

10. Average proportion (in %) of genes from the
recurrent parent
Percentage of homozygosity and
heterozygosity after selfing
The proportion of homozygotes for all the
gene in back cross=[(2m-1)/2m]n
m= no of back cross
n= no. of genes

11. Recovery of genes from RP
 Ave. recovery of RP = 1-(1/2)n+1, where n is the number
of backcrosses to RP
 The percentage recovery of RP varies among the
backcross progeny
 For example, in the BC3, if the DP and RP differ by 10
loci, 26% of the plants will be homozygous for the 10
alleles of the RP; remainder will vary.

12. Genetic consequences of backcross breeding
1. Reduction heterozygosis and increase in homozygosis
2. Increased similarity with the recurrent parent

13. RP- Recurrent parent;
DP- Donor parent;
BC- Backcross;
IRP- Improved recurrent parent.;
A. Stepwise transfer B. Simultaneous transfer; C. Simultaneous and stepwise transfer.

14. Applications of backcross breeding
1. Intervarietal transfer of simply inherited traits: characters
governed by one or two major genes – E.g.disease resistance,
seed color, plant height etc.
2. Intervarietal transfer of quantitative traits Quantitative characters
with high heritability can be transferred.
E.g. Earlyness, Plant height seed size, seed shape.
3. Interspecific transfer of simply inherited traits: Mostly disease
resistance from related species into a cultivated species.
Eg.(i) Leaf and stem rust resistance from Triticum timopheevii
T. monococcum, Aegilops speltoides and rye (S. cereale) to T.
aestivum
(ii) Black arm resistance from several Gossypium species to G.
hirsutum.

15. 4. Transfer of cytoplasm: B. C. method used to transfer cytoplasm from
one variety or species to another.
This is especial desirable in cases of Cytoplasmic or Cytoplasmic genetic
male sterility. E. g. Transfer of T. timopheevii cytoplasm to T.
aesticem.
5. Transgressive segregations:Back cross method may be modified to
obtain transgressive segregants.
6. Production of near isogenic lines:
 Isogenic lines are identical in their genotype, except for one gene.
 Such lines are useful in studying the effects of individual genes on
yield and other characteristics.
 Isogenic lines and easily produced using the back cross method.
7.Germplasm conversion: Conversion of photosensitive germplasm lines
(using as recurrent parent) to photo insensitive line (using a photo
insensitive line as a donor or non-recurrent parent.

16. Merits of Backcross method
1. The outcome of backcross programme is a well adopted variety
with superior traits, no need to conduct extensive yield tests.
2. Defects of well-adapted variety are removed without affecting its
adaptability and performance.
3. Much smaller population needed, when compared to pedigree
method.
4. Does not depend on environmental conditions, unless it is
required for selection of traits, hence 2-3 generation per year
can be grown.

17. Demerits
1. The performance of new variety is not significantly superior to
recurrent parent except for transferred trait
2. Problem of linkage drag – undesirable genes linked to gene being
transferred may also be transferred
3. Hybridization need to be carried out in each generation.
4. By the time new gene is being transferred to adopted variety, it
may be out of cultivation.

18. Concept of ‘linkage drag’
• Large amounts of donor chromosome remain even after many
backcrosses
• Undesirable due to other donor genes that negatively affect
agronomic performance
Donor/F1 BC1
c
BC3 BC10
TARGET
LOCUS
RECURRENT PARENT
CHROMOSOME
DONOR CHROMOSOME
TARGET
LOCUS
LINKED
DONOR
GENES

19. Marker-Assisted Backcrossing
May improve efficiency in three ways:
1) If phenotyping is difficult
2) Markers can be used to select against the donor parent in
the region outside the target
3) Markers can be used to select rare progeny that result from
recombination near the target gene

20. P1 x F1
P1 x P2
CONVENTIONAL BACKCROSSING
BC1
VISUAL SELECTION OF BC1 PLANTS THAT
MOST CLOSELY RESEMBLE RECURRENT
PARENT
BC2
MARKER-ASSISTED BACKCROSSING
P1 x F1
P1 x P2
BC1
USE ‘BACKGROUND’ MARKERS TO SELECT PLANTS
THAT HAVE MOST RP MARKERS AND SMALLEST %
OF DONOR GENOME
BC2