Genetic basis, recurrent, donor parent, linkage drag,procedure,dominant gene transfer, recessive gene transfer,stepwise, simultaneous transfer, application, merits, demerits,examples

1. Sub.:- Fundamentals of Plant Breeding
Course No. :- APB- 5211
Credit hours:- 3(2+1)
Lec. Topic :- Handling of Segregating generations.
Presented by:-
Lt. Roshan Parihar, Asstt. Professor
Deptt. of Genetics & PlantBreeding
Indira Gandhi Krishi Vishwavidyalaya
Raipur, C.G.
BTC College of Agriculture & Research Station
,Sarkanda, Bilaspur,(CG)-495001
The Backcross Breeding method

2. Handling of segregation
generation:-
The backcross breeding method

3. In early 20th century varieties bred thrugh pureline method with disease
resistant trait through intra-or inter- hybridization were inferior in yield or
quality to the generally accepted variety.
(Harlan and Pope (1922) suggested the back cross method by which
an undesirable allele at a particular locus is replaced by the desirable
allele in otherwise elite variety during the dev. of smooth awned variety of
barely (Manchuria) through the repeated backcrossing from local
manchuria barely genotypes.
 In other words, B.C. procedure conserves all good characteristics of a
popular adapted variety and incorporates a desirable character from
another variety.
The backcross method was frequently used by animal breeders to fix the
characterstics of breed.
Advent of Backcross breeding method

4. GENETIC BASIS OF BACKCROSS BREEDING
BC increases the frequency of desirable individual in population
F1
F2
1AA 2Aa
1aa
Population gradually
becomes identical to
recurrent parent
Normal Hybridization
Back Cross Method
𝑷𝒓𝒐𝒑𝒐𝒓𝒕𝒊𝒐𝒏 𝒐𝒇 𝒉𝒐𝒎𝒐𝒛𝒚𝒈𝒐𝒖𝒔 𝒊𝒏𝒅𝒊𝒗𝒊𝒅𝒖𝒂𝒍𝒔 = 𝟐 𝒎
𝟐 𝒎 − 𝟏 𝒏
50%
Genes
from
both
Reapeated backcrossing of F1 with Recurrent parent
or Deficient parent would gradually increase the
genetic component from recurrent parent with the
proportionate elimination of gene component from
donor parent

5. Genetic basis of backcross breeding contd..
In both the situations i.e. selfing as backcrossing proportion of complete
homozygous individuals increases with the same rate
m = number of generation of back crossing or selfing
n = number of gene pair for which parent carry contrasting
allelles
Both m and n are powers in the formula
n
(Allard, 1960)
Gen. of backcrossing Avg. recovery of genes from
Recurrent parent
Avg. recovery of genes from
Donor parent
F1 50 50
BC1 75 25
BC2 87.5 12.5
BC3 92.75 6.25
BC4 96.875 3.125
BC5 98.4375 1.5625
BCm 1-(1/2)m+1 (1/2)m+1

6. Limitations on the effectiveness of Backcross breeding
Linkage drag: It leads to transfer of undesirable gene (b) in in recessive
forms.
• The chance of breaking linkage drag (undesirable genes) is more
with BC than selfing.
• Ex: ‘A’ is desirable gene and linked to undesirable gene ‘b’, desirable
gene has to transferred from donor parent to well adapted variety.
F1
Desirable
gene variety
Adapted variety
A and a have the
tendency to inherit
together to make it
difficult to obtain AB
combination.
Since gene B is
reintroduced with
each back crosses.
Probability of eliminating of b
gene
= 𝟏 −(𝟏 − 𝑷) 𝒎+𝟏
P = recombination fraction
m = number of backcross

7. No. of Backcrosses: The main aim to BC method is to regain the
genotype of recurrent parent with additional gene content.
• The wild germplasm is usually too diverse to provide opportunities
for free recombination.
• It leads to do more additional backcross generations.
• The use of tightlylinked molecular markers helpto identify the
recurrentparent in 2-3 backcross while 10 BC gen. are required in
conventionalbreeding.
• Tansley (1989) suggented that MABB will reduce linkage drag at least
10fold
• eg . Used in tomato Wild sp .L pennellii X Cultivated tomato for
improvement of soluble solids

8. The behaviour of character under transfer of Backcrosses: The main
aim to BC method is to regain the genotype of recurrent parent with
additional gene content.
• The qualitative genes can easily be screend out by their phenotypic
performance.
• The targeted gene should have high heritability percentage(h2).
• The gene under interest should not be influenced by modifier genes.
• Tansley (1989) suggested that MABB will reduce linkage drag at least
10fold
• eg . Used in tomato Wild sp .L pennellii X Cultivated tomato for
improvement of soluble solids

9. BACK CROSS BREEDING
• Back cross is the crossing of F1 with either of its parents is called as
back cross
• Crossing of F1 with its recessive parent is known as test cross
• Recurrent parent: The parent which is occurs repeatedly in the crossing
Programme, it is superior in all traits except trait of interest (Deficit parent)
• Donor parent: The parent which used only once in the breeding
programme and donates desirable genes, it is inferior to recurrent
parent in all the traits except trait of interest
• Recipient parent: Well adapted, high yielding variety, lacking one or two
traits hence receives these lacking genes from other variety.
• Non recurrent parent: The donor parent.

10. REQUIREMENTS
• Good recurrent parent
• Suitable good donor
• High expressivity of traits
• Character should be highly heritable
• Simple testing techniques
• Recovery of recurrent parent in minimum back crosses

11. Applications of backcross breeding
Applicable to cross and self pollinated crops
 Inter varietal transfer of simply inherited traits: (disease
resistances and colour)
 Linkage drag: failure of transfer of simply inherited traits like disease
resistance by B C method due to tight linkage between the genebeing
transferred and some other undesirable genes
 Inter varietal transfer of Quantitative traits: (earliness, Plant
height, Seed size and shape, yield)
 Interspecific transfer of simply inherited traits: leaf and stem rust
resistance from T. timopheevii, black arm from Gossypium species
to G. hirsutum

12. 1. Transfer of cytoplasm: (CMS and CGMS) T. timopheovii to T.
aestivum
2. Transgressive segregation: 1. the F1 may be back crossedto
1 ot 2 time to recurrent parents
1. two or more recurrent parents may be used in BCmethod
3. Production of isogenic lines: isogenic lines are identical in
their genotypes, expect for one gene
4. Germplasm conversions: conversion of photosensitive lines
(using as recurrent parent) in to photo insensitive lines (donor
parents)

13. F1 50%
Stem res.Susceptible
Non recurrent parent ‘B’
(Donor) (Male)
Recurrent parent ‘A’
(Def.) (Female)
Recurrent
parent
rr
rr RR
BC6 (Rr)
(99.218%)
BC5
(98.438 :1.562)
Transfer of dominant gene
RR
No. Seg.
Rr
Segre.
rr
reject
Selfing
Sel. These
RR families

16. 1.Stepwise transfer, In 1st step Recurrent parent is converted through donor, in 2nd
step converted recurrent parent is again converted through new donor and so on.
2.Simultaneous transfer, In this method cross between recurent and donor is
created and F1 is crossed with new donor by three way cross and Subsequently BC
generation are made through recurrent parent cross.
3.Stepwise transfer but parallel transfer, separate BC program for the
transfer of each character separately at the same time and F1s are crossed separtely and
complex F1 is created and and F2 genertion id handled through Pedigree method.
Transfer of two or more traits into a single Recurrent parent

19. Merits:
• New variety is nearly identical with that of recurrent variety except the
gene of interest
• This is useful method to transfer oligogenes (disease resistance) and
polygenes (oil and protein content)
• Extensively used for the development of varieties with multiple disease
resistance; this method is used to develop NILs
• The male sterility and fertility restoration genes can be transferred through
this method
• Interspecific gene transfer can achieved through this method only.

20. Merits contd..
• It is genetically precise and provides max. genetic control during crop imp.
• The varieties developed through this method does not require extensive
field testing.
• The method is free from the enviromental effect as the character is fully
expressed. i.e. off season nursery can be used.
• This method is a ideal solution to utilize unadapted germplasm which
seems to unproductive but has res. Genes.

21. • The newly developed variety cannot be superior to the recurrent parent
except trait of interest
• Involve lot of crossing work, which is costly and time consuming.
• Possibility of linkage drag.
Demerits
Achievements
Cotton: 170-co-2, 134-co-2m, V797, Digvijay
Wheat: Robin, K1, Blue bird, Tobari, HS-19
Bajra (Pearl millet): MS-521A, MS-541A, MS-570A

22. Parameters Pedigree Bulk Backcross
Application SP and CP SP SP, CP and Asexual
Crossing Only once Only once Repeated
Selection Artificial Both natural and artificial Artificial
F2 to be evalu1ated Smaller than bulk Larger than pedigree Smaller
Maintenance of
record
Maintained Not maintained Not maintained
Effectiveness
Effective for oligo and poly
genes
Effective for oligo and poly
genes
More effective for oligo and
less for poly gene
Testing Extensive Extensive Not required Extensive
Time taken 14-15 years longer than pedigree 7-8 years
Breeding
procedure
Same for dominant and
recessive
Same for dominant and
recessive
Differs for dominant and
recessive
Adaption Narrow wider Like parent
Use Widely used less popular Widely used
Features of new Different from both parent Different from both parent
Identical with recurrent
parent

23. THANK YOU