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#backcross breeding

3 public questions tagged with this topic.

Plant Breeding 3

Backcross breeding is commonly used for transfer of:

Backcrossing excels at transferring discrete, simply inherited traits governed by one or few genes where phenotype reliably identifies presence of donor allele. Disease resistance genes such as Sr2 for stem rust, Pi54 for blast, and Lr genes in wheat are typically dominant monogenic traits with major effect and high heritability, easily screened by inoculation in each backcross generation using pathogen isolates. Introgression of quantitative, polygenic traits like yield is inefficient because many loci would need simultaneous transfer, epistasis complicates selection, and linkage drag would impair elite background. Yield itself relies on hundreds of QTLs and strong G×E interaction, making backcross recovery less valuable. Therefore breeders employ backcross primarily for major gene resistances, male sterility, quality traits like waxy endosperm, and single-gene herbicide tolerance, often complementing with marker-assisted foreground selection using tightly linked markers to ensure gene presence. Typical protocol includes recurrent parent as susceptible elite cultivar needing improvement, donor as resistant landrace or wild relative, repeated inoculation and marker screening each backcross to maintain resistance allele while restoring agronomic superiority and quality characteristics.

Ref: Brar DS & Khush GS. Alien introgression in rice, Plant Molecular Biology 1997. Collard & Mackill 2008 MAS review

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Plant Breeding 3

The parent repeatedly used in backcrossing is called:

In a backcross program two parents play distinct roles. Donor parent, also called non-recurrent parent, carries elite allele missing in adapted cultivar but is otherwise agronomically inferior. Recurrent parent is the high-yielding, widely adapted cultivar targeted for improvement, used as female or male in every backcross to restore its genome. After initial F1 = donor × recurrent, selected progeny possessing donor allele is crossed again to same recurrent parent, producing BC1F1, then BC2F1, and so on. Repeated use elevates proportion of recurrent genome from 75% in BC1 to 87.5% in BC2, 93.75% in BC3, following 1-(1/2)^(n+1). Recurrent parent is thus recipient background that must be recovered fully, while donor segment around target locus shrinks through recombination. Choice of recurrent parent determines final cultivar adaptation, so elite lines with broad stability and market acceptance are preferred to maximize commercial impact of introgression. Cytoplasmic background usually comes from recurrent parent when used as female in final backcross, ensuring nuclear-cytoplasmic compatibility and preserving cytoplasmic male sterility systems or organelle encoded traits important for hybrid seed production.

Ref: Hawkes JG. The genetics of backcross. Biology LibreTexts: Recurrent and donor parent definitions

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Plant Breeding 3

Backcross breeding is mainly used to:

Backcrossing is designed to rectify a specific defect in an otherwise elite, adapted cultivar without altering its overall genetic background. A donor parent contributes one or a few desirable alleles, such as a dominant disease resistance gene Yr15, Bt, or bacterial blight gene Xa21, while recurrent parent provides superior yield, quality, and adaptation encoded by hundreds of favorable QTLs. By repeatedly crossing hybrid progeny carrying the target gene back to recurrent parent and selecting for that gene each cycle, breeder introgresses a small donor chromosomal segment. Unlinked donor genome decays by 50% each backcross, restoring elite genome to >99% by BC6. This precision contrasts with pedigree methods that create new variability through broad recombination. It is ideal for adding oligogenic traits to established varieties, near-isogenic line development, and transgene introgression while preserving linkage blocks responsible for elite performance and G×E stability. At molecular level, foreground selection with SSR or SNP markers flanking target gene tracks introgression, while background selection with genome-wide markers minimizes donor chromosome retention and reduces linkage drag around inserted locus significantly.

Ref: Allard RW. Principles of Plant Breeding, Chapter 15 Backcross. Hospital 2005, Nature Reviews Genetics

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