Back cross method

Definition:
In breeding, a cross of a hybrid with one its parents or with a genetically equivalent
organism. In genetics, a cross of a hybrid with a homozygous recessive, also known as test
cross.
Recurrent parent: The adapted parent, to which the allele is being added, enters into each
backcross. In other words, parent to which hybrid material is crossed in a backcross.
Non-recurrent parent: The parent with the superior characters enters into the initial cross
but does not enter into the backcross, also donor parent.
In backcross method of breeding, the hybrid and the progenies in subsequent
generations are repeatedly backcrossed to one of the parents. As a result, the genotype of the
backcross progeny becomes increasingly similar to that of the recurrent parent. The objective
of backcross method is to improve one or two specific defects of a high yielding variety.
Pre-requisite for back cross breeding
1. A suitable recurrent parent must be available which lacks in one or two
characteristics.
2. A suitable donor parent must be available
3. The character to be transferred must have high heritability and preferably it should be
determined by one or two genes.
4. A sufficient number of back crosses should be made so that the genotype of recurrent
parent is recovered in full.
Application of back cross method
This method is commonly used to transfer disease resistance from one variety to another.
But it is also useful for transfer of other characteristics.
1. Inter-varietal transfer of simply inherited characters
E.g. Disease resistance, seed coat colour
2. Inter-varietal transfer of quantitative characters.
E.g. Plant height, Seed size, Seed shape.
3. Interspecific transfer of simply inherited characters
E.g. Transfer of disease resistance from related species to cultivated species.
E.g. Resistance to black arm disease in cotton from wild tetraploid species into
G. hirsutum
4. Transfer of cytoplasm
This is employed to transfer male sterility. The female parent will be having the sterile
cytoplasm and recurrent parent will be used as male parent.
E.g. Sesamum malabariucum (Female parent) x S. indicum (recurrent parent)
Procedure for backcross method
The Plan of backcross method would depend upon whether the gene being transferred
is recessive or dominant. The plan for transfer of a dominant gene is simpler than that for a
recessive gene.
Original cross Disease resistant cultivar Adapted cultivar A
RR rr
1st Backcross F1 Cultivar A
Rr rr
50% genes from A
2nd Backcross BC1 Cultivar A
Rr:rr rr
75% genes from A
3rd Backcross BC2 Cultivar A
Rr:rr rr
87.5% genes from A
4th Backcross BC3 Cultivar A
Rr:rr rr
93.75% genes from A
BC4
Rr:rr
96.875% genes from A

Self Rr plants from BC4 to

obtain plants homozygous
for RR

1RR:2Rr:1rr

Fig: Procedure for a backcross in which a dominant allele for disease resistance (R) is
transferred from a disease resistant cultivar to an adapted cultivar A. The resistant donor
cultivar is crossed to the adapted recurrent cultivar A, and the F1 backcrossed to cultivar A.
The BC1 generation from this cross will be segregating for disease resistance (Rr:rr). The Rr
plants may be identified from the rr plants by inoculating the seedling plant with the disease
pathogen and observing weather plants exhibit the resistant or the susceptible disease
reactions. Only Rr (resistant) plants are backcrossed to A in the second and succeeding
backcross generation. After the final backcross, the heterozygous (Rr) plants are slefed one
generation to obtain the homozygous (RR) and the heterozygous (Rr) resistant plants.
Progeny tests of the resistant (RR and Rr) plants are grown to identify the homozygous (RR)
from the heterozygous (Rr) plants, so that lines pure for resistance may be established.
Merits of Backcross Method
 The genotype of the new variety is nearly identical with that of the recurrent parent,
except for the genes transferred. Thus the outcome of a backcross programme is
known beforehand, and it can be reproduced any time in the future.
 It is not necessary to test the variety developed by the back cross method in extensive
yield tests because the performance of the recurrent parent is already known. This
may save upto 5 years time and a considerable expense.
 The backcross programme is not dependent upon environment, except for that needed
for the selection of the character under transfer. Therefore, off-season nurseries and
green - houses can be used to grow 2-3 generation each year. This would drastically
reduce the time required for developing the new variety.
 Much smaller population are needed in the backcross method than in the case of
pedigree method.
 Defects, such as, susceptibility to disease, of a well-adapted variety can be removed
without affecting its performance and adaptability. Such a variety is often preferred
by the farmers and the industries to an entirely new variety because they know the
recurrent variety well.
 This is the only method for interspecific gene transfers.
 It may be modified so that transgressive segregation may occur for quantitative'
characters.
Demerits of Backcross Method
 The new variety generally cannot be superior to the recurrent parent, except for the
character that is transferred.
 Undesirable genes closely linked with the gene being transferred may also be
transmitted to the new variety.
 Hybridization has to be done for each backcross. This is often difficult, time taking
and costly.
 By the time the backcross is over, the recurrent parent may have been replaced by
other varieties superior in yielding ability and other characteristics.