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#recurrent parent

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Plant Breeding 3

Each backcross generation recovers recurrent parent genome by:

Genomic recovery after backcrossing follows Mendelian expectation of halving unlinked donor genome each cycle. F1 contains 50% recurrent and 50% donor. When F1 is crossed to recurrent parent, gametes from F1 carry on average half donor alleles; progeny BC1F1 averages 75% recurrent, gaining 25% increment which equals half the remaining donor proportion. In each subsequent backcross, donor fraction halves again: BC2F1 87.5%, BC3F1 93.75%, BC4F1 96.875%, achieving rapid return to recurrent background. Formula for recurrent proportion after n backcrosses = 1 - (1/2)^(n+1). Recovery rate pertains to genome unlinked to target; linked donor segment around selected gene persists longer due to linkage drag and requires extra recombination. Marker-assisted background selection with genome-wide SSRs accelerates recovery beyond theoretical expectation by selecting BC individuals with highest recurrent alleles, often reaching 98% by BC3 and reducing need for six backcrosses. With marker-assisted background selection, breeders can achieve recurrent parent genome recovery above 98 percent by BC3, reducing generations needed and saving time compared to conventional phenotypic recurrent genome restoration relying solely on visual similarity assessment.

Ref: Hospital F. 2005 Genetics 160: 1071 – Size of donor genome. Allard, Chapter 15

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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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