Backcross Breeding

1
Backcross Breeding
1

• Essentially a method for improving an
  established variety that is deficient in only one
  or a few characteristics
• The backcross generations converge on a single
  genotype
• Only breeding method in which the results are
  both predictable and repeatable

2
Requirements
2

• A satisfactory recurrent parent, usually an
  established cultivar
• A donor parent with a desirable characteristic
• A sufficient number of backcrosses to
  reconstitute the recurrent parent

3
Implementation
3

• A cross is made between the Donor and
  Recurrent parent, then the F1 and subsequent
  generations are crossed (backcrossed) to the
  recurrent parent
• The backcross generations converge on a single
  genotype
• The genetic contribution of the Donor parent
  will be halved each generation

4
Recovery of recurrent parent
4

• Generation Recurrent parent
• F1 50
• BC1 75
• BC2 87.5
• BC3 93.75
• BC4 96.87
• etc...
• same rate as homozygosity in a selfing
  series

1- (1/2)m, where m number of generations of
backcrossing or selfing
5
Proportion of homozygous genotypes
5

• Example 1
• Assuming 3 loci and 3 backcrosses
• (23 - 1) / (23)3 .669
• Example 2
• Assuming 10 loci and 5 backcrosses
• (25 - 1) / (25)10 .728

(2m - 1) / (2m)n m number of
generations of backcrossing or selfing n number
of heterozygous loci in F1
6
Rate of inbreeding
6
7
Features of Backcross method
7

• Provides breeder a high degree of control
• Traits to be improved can be described in advance
• It is repeatable
• Extensive field trials are not required
• Few notes (record keeping)

8
Negative Features
8

• Improved variety is same as recurrent
  parent except for trait transferred
• Minimal recombination
• Linkages to deleterious genes
• Not effective for quantitative traits

9
Backcrossing dominant gene for rust resistance
9

• Parker P1 (rr) x Baart P2 (RR)
• F1 (Rr) (resistant)
• BC1 Parker (rr) 1/2 rr 1/2 Rr (inoculate )
• BC2 Parker (rr) 1/2 rr 1/2 Rr (inoculate )
• BC3 Parker (rr) 1/2 rr 1/2 Rr (inoculate )

After BC, then self
10
Backcrossing a recessive gene for rust resistance
11

• Recurrent P1 (RR) x Donor P2 (rr)
• F1 (Rr) (suseptable)
• BC1 Recurrent (RR) 1/2 RR 1/2 Rr (suseptable
  )
• How to identify the heterozygotes carrying the
• desirable recessive gene for resistance?
• 1) Progeny test
• 2) Blind backcross

11
Blind backcrossing
12

• Recurrent P1 (RR) x donor P2 (rr)
• F1 (Rr) (suseptable)
• BC1 RCP (RR) 1/2 RR 1/2 Rr
• BC2 RCP (RR) 3/4 RR 1/4 Rr
• BC3 RDC (RR) 7/8 RR 1/8 Rr (self and
  inoculate to recover rr
  individuals)

12
Backcrossing more than one gene
13

• Recurrent (P1) X Donor (P2)
• AABB aabb
• F1 AaBb
• AABB AABB AaBB AABb AaBb
• (only 1/4 of plants contain both a and b
  alleles)

13
Number of plants required
14
14
Marker facilitated backcrossing
15

• Two objectives
• Transfer of trait
• Recovery of recurrent parent genotype
• Can use selection based on phenotypes in early
  backcross generations to shift population towards
  recurrent parent
• Molecular markers can also be used to
  characterize individuals in early generations.

15
F2 population
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• Indiv M1 M2 M3 M4
• 1 R H D R 5/8
• 2 D H D R 3/8
• 3 R H D H 4/8
• 4 R H R R 7/8
• R homozygous for recurrent parent genotype (2)
• H heterozygous (1)
• D homozygous for donor parent genotype (0)

16
F2 and BC1 distributions
17
Backcross
15 gt90RCP
F2
3 gt90RCP
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Backcross generations
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• P1 (BB) x P2 (bb)
• F1 (Bb) x P1 (BB)
• Backcross progeny
• BB Bb 1/2 1/2 a d Mean
  1/2a 1/2d
• Variance 1/4a2 1/4d2 - 1/2ad
• Note if d0, then Variance of BC is 1/2 that of
  F2

homozygous for recurrent parent genotype
homozygous for recurrent parent genotype