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Backcrossing

Backcrossing is a breeding technique in genetics and agriculture that involves repeatedly crossing a hybrid offspring with one of its parents, referred to as the recurrent parent, to transfer one or a few desirable traits—such as disease resistance—from a donor parent into the elite genetic background of the recurrent parent while minimizing the introduction of unwanted donor genome.^[1] This method ensures that the resulting lines retain the superior agronomic performance of the recurrent parent, typically recovering over 96% of its genome by the fourth backcross generation and approaching 99.2% by the sixth.^[2] The process of backcrossing begins with an initial hybridization between the donor parent, which carries the target trait, and the recurrent parent, yielding F1 progeny with approximately 50% genetic contribution from each.^[1] Progeny exhibiting the desired trait are then selected and backcrossed to the recurrent parent, a step repeated for 4 to 7 generations depending on the trait's dominance and the required recovery level.^[3] For dominant traits, selection occurs directly in backcross populations; for recessive traits, self-pollination to the F2 generation may be necessary after each backcross to identify and fix the allele before continuing.^[3] Once the target trait is stably introgressed, final lines are often selfed or intercrossed to achieve homozygosity and evaluate performance.^[1] Backcrossing has been a foundational tool in plant breeding since the early 20th century, enabling the incorporation of beneficial alleles from wild or unadapted germplasm into elite cultivars to improve traits like yield, quality, and environmental stress tolerance.^[4] In modern applications, it is extensively used in crops such as rice,^[5] maize,^[6] and tomato^[7] to introgress transgenes or quantitative trait loci (QTL) for enhanced disease resistance and nutritional value. Beyond plants, backcrossing facilitates the creation of congenic strains in animal models, such as mice, for studying gene functions and histocompatibility, and supports genetic improvement in livestock by reducing linkage drag around target loci.^[4] Advancements in molecular biology have significantly enhanced backcrossing efficiency through marker-assisted selection (MAS), which uses DNA markers to track the target trait and select for recurrent parent genome recovery, often saving up to two generations compared to phenotypic selection alone.^[4] This approach also minimizes the retention of undesirable donor segments, known as linkage drag, and enables the development of near-isogenic lines (NILs) for precise QTL validation.^[4] In contemporary genetics, backcrossing combined with recurrent selection schemes further aids in fine-mapping complex traits and tapping exotic genetic diversity for sustainable breeding programs.^[4]

Overview

Definition

Backcrossing is a specialized breeding technique in genetics and agriculture that involves the repeated crossing of a hybrid offspring with one of its parents, referred to as the recurrent parent, to transfer a specific desirable gene or trait from the other parent, known as the donor parent, while progressively recovering the genetic makeup of the recurrent parent.^[8] This method is particularly valued for its ability to introduce targeted genetic improvements without substantially altering the established traits of an elite variety.^[1] The core purpose of backcrossing is to introgress a single trait into an elite genetic background, minimizing disruptions to the rest of the genome and preserving the recurrent parent's superior qualities, such as yield, adaptability, or disease resistance.^[9] In this approach, the recurrent parent is typically an established, high-performing line, while the donor parent provides the novel allele or trait of interest, such as pest resistance or a quality enhancement.^[3] The basic process commences with an initial hybridization between the donor and recurrent parents to produce F1 hybrids, followed by multiple backcross generations where progeny carrying the desired trait are selectively crossed back to the recurrent parent.^[1] These backcross progeny are designated as BC1 for the first backcross generation, BC2 for the second, and so forth, with typically 6 to 8 generations required to achieve over 99% recovery of the recurrent parent's genome.^[10] Through this iterative selection and crossing, the proportion of the donor parent's genome diminishes rapidly, allowing for targeted trait incorporation.^[11]

Historical Development

Backcrossing, as a deliberate breeding technique, emerged from informal practices observed among 19th-century farmers and early plant breeders who repeatedly crossed hybrids to parental types to stabilize desirable traits in crops such as wheat and barley. In France, Louis de Vilmorin advanced selection methods through progeny testing starting in the 1850s to maintain uniformity and enhance specific characteristics like seed quality in beets and cereals.^[12]^[13] The method gained formal recognition in the early 20th century through genetic research, initially in model organisms. Thomas Hunt Morgan, in his 1910 studies on Drosophila melanogaster, employed backcrossing to trace inheritance patterns, such as sex-linked traits, by repeatedly crossing hybrid offspring to parental strains, which helped establish chromosomal theory and linkage mapping. In plant breeding, geneticists like Edward M. East applied similar recurrent crossing in corn improvement programs around the 1910s, using backcrosses alongside inbreeding to recover elite parental genomes while incorporating traits like disease resistance, contributing to the foundational work on heterosis.^[14] The technique was explicitly formalized for crops by Harry V. Harlan and Merritt N. Pope in 1922, who demonstrated its value in small-grain breeding through repeated backcrosses to introgress traits like awn smoothness in barley without disrupting overall varietal performance.^[15] A key milestone occurred in the 1910s and 1920s with widespread adoption at U.S. agricultural research stations, where backcrossing facilitated hybrid corn development by enabling efficient trait transfer into inbred lines, as seen in programs at the Connecticut Agricultural Experiment Station under Donald F. Jones.^[16] Similarly, it supported wheat improvement efforts at USDA stations, accelerating the creation of rust-resistant varieties through recurrent selection.^[17] Following World War II, backcrossing expanded significantly during the Green Revolution, particularly through Norman Borlaug's wheat breeding in Mexico during the 1950s and 1960s, where it was used to introgress stem rust resistance genes from donor varieties into high-yielding backgrounds, resulting in semi-dwarf cultivars that boosted global production.^[18] This integration into large-scale programs underscored backcrossing's role in addressing food security challenges. By the late 20th century and into the 2020s, backcrossing evolved historically toward precision applications, with the introduction of molecular markers in the 1990s enabling marker-assisted backcrossing (MABC) to accelerate genome recovery and minimize linkage drag, as evidenced in ongoing crop improvement initiatives up to 2025. In the 2020s, backcrossing has integrated with genomic prediction and gene editing technologies to further enhance precision and efficiency in trait transfer.^[19]^[20]^[21]

Genetic Principles

Mechanism

Backcrossing relies on the principles of meiosis and genetic recombination to facilitate the introgression of a specific trait from a donor parent into the genome of a recurrent parent while progressively recovering the recurrent parent's genetic background. During meiosis in the hybrid offspring, homologous chromosomes pair and undergo recombination, leading to the segregation of alleles. In each backcross generation, approximately half of the donor parent's genome is replaced by alleles from the recurrent parent through this random segregation process, as gametes inherit one allele from each parent at each locus.^[22]^[23] The effectiveness of backcrossing is enhanced by targeted selection in each generation, where progeny are screened to retain the desired donor trait while favoring the recurrent parent's alleles elsewhere in the genome. Phenotypic selection identifies individuals expressing the target trait based on observable characteristics, such as disease resistance, while genotypic selection uses molecular markers to detect the presence of the donor allele at the target locus and the recurrent parent's alleles in the background. This dual selection minimizes the retention of undesirable donor genetic material, ensuring that the introgressed segment remains focused around the target gene.^[24]^[25] Over successive backcross generations, the proportion of the recurrent parent genome increases predictably due to the halving of donor-derived heterozygosity with each cycle. After n backcross generations, the expected proportion of the recurrent parent genome is approximately $1 - \frac{1}{2^{n+1}}, reflecting the exponential reduction in donor alleles through repeated segregation without selection for background recovery. This formula assumes random chromosome assortment and no linkage effects across the entire genome.^[26]^[10] Several genetic factors influence the efficiency of this process, particularly linkage drag and recombination frequency near the target locus. Linkage drag occurs when undesirable donor alleles are physically linked to the target gene, resulting in their co-inheritance and potential reduction in the recipient's fitness unless broken by recombination. The frequency of recombination in the chromosomal region surrounding the target locus determines how readily these linked segments can be separated; higher recombination rates allow for finer resolution of the introgressed segment, reducing drag, while low-recombination regions prolong the retention of unwanted donor DNA.^[27]^[28]

Genome Recovery

In backcrossing, the genome of the recurrent parent is progressively recovered through successive generations, as the progeny are repeatedly crossed back to the recurrent parent. The expected proportion of the recurrent parent genome (RPG) in BC_n progeny follows the quantitative model $1 - \left(\frac{1}{2}\right)^{n+1}, where n represents the number of backcross generations. This formula arises from the halving of the donor genome contribution at each backcross due to segregation during meiosis. For instance, BC_1 progeny recover approximately 75% of the recurrent parent genome, while BC_6 progeny achieve about 99.2% recovery.^[24] To fix the target trait in a homozygous state while further maximizing RPG recovery, selfing is commonly applied in later generations following the final backcross. For example, after reaching BC_4F_1, selected plants are self-pollinated to generate BC_4F_2 progeny, where homozygous individuals for the target trait (e.g., 25% homozygous resistant for a dominant trait) can be identified and advanced. Intercrossing among selected BC_n individuals may also be employed in certain programs to promote additional recombination, aiding in the reduction of donor segments and enhancement of recovery. These steps ensure the target trait is stably incorporated without excessive donor influence.^[29] Genome recovery is precisely measured using molecular markers, such as simple sequence repeats (SSRs) or single nucleotide polymorphisms (SNPs), which allow tracking of donor versus recurrent parent chromosomal segments across the genome. By genotyping progeny at polymorphic loci flanking the target trait and throughout the genome, breeders can quantify the exact RPG proportion and select individuals with minimal donor introgression. This approach reveals deviations from theoretical expectations due to recombination patterns.^[30] However, incomplete genome recovery remains a limitation, as random segregation and physical linkage preserve some donor chromatin, often leading to linkage drag—where undesirable donor alleles tightly linked to the target locus persist and negatively impact agronomic performance. In large genomes, such as those of many crop plants, achieving near-complete recovery (over 99%) typically requires 6-8 generations of backcrossing, even with marker assistance, to sufficiently dilute residual donor segments.^[22]^[31]

Plant Applications

Breeding Methods

Backcrossing in plant breeding involves crossing a hybrid progeny with the recurrent parent to introgress one or a few desirable traits from a donor parent into an elite genetic background. The process starts with an initial hybridization between the donor (carrying the target trait, such as disease resistance) and the recurrent parent (elite cultivar), producing F1 progeny. Progeny expressing the desired trait are selected and backcrossed to the recurrent parent, with this step repeated for 4 to 7 generations to achieve high recurrent parent genome recovery, typically over 96% by the fourth generation.^[3] For dominant traits, selection can occur directly in backcross populations using phenotypic evaluation. For recessive traits, self-pollination to the F2 generation after each backcross is required to identify homozygous individuals carrying the allele, followed by further backcrossing. In self-pollinating crops like wheat or rice, this allows fixation of the trait through homozygosity. After sufficient backcrosses, final lines are often selfed or intercrossed to develop homozygous varieties and evaluate agronomic performance.^[3]^[10] Marker-assisted backcrossing (MABC) enhances efficiency by using DNA markers for selection. It includes foreground selection (confirming the target gene), recombinant selection (reducing donor segments around the gene via flanking markers), and background selection (maximizing recurrent parent genome recovery using genome-wide markers). This approach often reduces the number of backcross generations to 2-3, with population sizes of 400+ plants per generation, and is particularly useful in crops like rice and maize.^[5]^[10]

Advantages and Challenges

Backcrossing offers key advantages in plant breeding by enabling the precise transfer of beneficial traits, such as disease or pest resistance, into elite cultivars while preserving their superior agronomic qualities. It efficiently recovers the recurrent parent's genome, reaching 99.2% similarity after six generations, which minimizes disruptions to yield, quality, or adaptation. With MABC, the process is accelerated, often saving 1-2 generations compared to conventional methods, and reduces linkage drag—the retention of unwanted donor genes—by selecting recombinants near the target locus. This makes it ideal for incorporating alleles from wild relatives or unadapted germplasm, enhancing traits like submergence tolerance or nutritional quality without extensive breeding from scratch.^[5]^[3] However, challenges include the need for multiple generations, which can be time-consuming in crops with longer cycles, such as perennial plants. Linkage drag remains a risk if markers are not closely linked to the target gene, potentially introducing undesirable traits that affect performance. For recessive traits, additional selfing steps increase complexity and time. Initial costs for marker development and genotyping (approximately US$0.30-1.00 per data point) can be high, though they decrease with reuse. Additionally, backcrossing is most effective for single or few genes and less suitable for polygenic traits, where other methods like recurrent selection may be preferred.^[5]^[10]

Specific Examples

In rice breeding, backcrossing has been widely used to introgress bacterial blight resistance conferred by the Xa21 gene from donor lines into elite varieties like IR64 and Swarna. Using MABC, the International Rice Research Institute (IRRI) developed resistant lines in 3-4 generations, recovering over 98% of the recurrent genome and reducing submergence susceptibility with the Sub1 QTL, improving yields in flood-prone areas of Asia.^[5] For maize, backcrossing facilitated the development of quality protein maize (QPM) by transferring opaque-2 modifier genes from donor parents into elite tropical lines. Conventional backcrossing over 5-6 generations, later enhanced by MABC, increased lysine and tryptophan content by 50-100% while maintaining yield, benefiting nutrition in sub-Saharan Africa and Latin America. Insect resistance against stem borers has also been introgressed using MABC in tropical maize hybrids.^[5]^[10] In tomato, backcrossing introgressed the Tm-2a gene for tomato mosaic virus resistance from wild relatives into commercial cultivars. Multiple backcrosses (up to 7) to recurrent parents like 'VFNT' achieved near-isogenic lines with over 99% elite genome recovery, preserving fruit quality and yield while conferring durable resistance, as demonstrated in programs since the 1970s.^[5]

Animal Applications

Breeding Methods

In animal breeding, backcrossing protocols are adapted to accommodate the viviparous reproduction and extended lifespans of livestock species, such as cattle, swine, and sheep, which impose significant constraints compared to self-pollinating plants. The process begins with an initial cross between a donor animal carrying the desired trait and the recurrent parent of the elite breed, followed by repeated matings to the recurrent parent to recover its genetic background while retaining the target allele. To mitigate the challenges of natural mating limitations, artificial insemination (AI) is routinely integrated, allowing precise control over semen deposition from selected recurrent sires and enabling the dissemination of genetics across large herds without physical proximity. Embryo transfer (ET) further accelerates progress by harvesting multiple embryos from a superior donor female after superovulation and hormonal synchronization, then implanting them into surrogate recipients, thereby multiplying offspring per generation and shortening effective timelines. These techniques are particularly valuable in species like cattle, where ET combined with AI can produce dozens of progeny from one female in a single cycle.^[32]^[33] Due to the protracted generation intervals—typically 1-4 years in livestock, with cattle averaging around 4 years—backcrossing programs are generally limited to 3-5 generations to balance genome recovery with practical feasibility and costs. This contrasts sharply with plant breeding, where shorter cycles allow for more extensive backcrosses. Each backcross generation aims to increase the proportion of the recurrent parent's genome by approximately 50% additively, referencing the core mechanism of allele substitution and linkage drag reduction. Protocol adjustments emphasize early screening for the target trait to discard non-carriers, often using phenotypic evaluation or molecular markers in later stages, ensuring efficient progression despite fewer iterations. In practice, 3-5 backcrosses suffice to achieve over 93% recovery of the recurrent genome after three generations, approaching 98% after five, minimizing donor introgression while fixing the desired trait. Selection integration within backcrossing relies heavily on pedigree tracking to monitor inheritance patterns and ensure fidelity to the recurrent parent, complemented by progeny testing to validate trait performance across offspring. For quantitative traits like milk yield in dairy cattle, progeny testing involves evaluating daughters' production records from AI-sired litters, providing reliable estimates of sire genetic merit after one or more lactations. This method, standard in Holstein breeding programs, allows selection of backcross progeny that not only carry the donor trait but also maintain or enhance elite production metrics. Pedigree records, often maintained via centralized databases, facilitate tracing multi-generation lineage to avoid inbreeding and confirm recurrent parent similarity.^[34]^[35] Mating schemes in animal backcrossing emphasize controlled environments within herds or flocks to maximize recurrent parent resemblance, using fenced pastures or indoor facilities for supervised pairings. Sires from the recurrent breed are selected for high genetic value and rotated to prevent relatedness, with AI enabling widespread use of top individuals across dispersed groups. In swine or poultry flocks, group housing facilitates natural mating post-initial AI/ET, but all subsequent backcrosses prioritize recurrent breed dominance through sire choice. These schemes incorporate synchronization protocols for estrus in females to align breeding windows, optimizing litter sizes and minimizing downtime between generations. Overall, such adaptations ensure backcrossing remains viable for introgressing traits like disease resistance or growth efficiency into commercial livestock lines despite reproductive hurdles.^[36]^[32]

Advantages and Challenges

Backcrossing in animal breeding offers several advantages, particularly for introducing specific desirable traits into established lines while maintaining the overall genetic background of elite or high-value animals. One key benefit is the rapid fixation of single traits, such as disease resistance, into recurrent parent lines through successive generations, allowing breeders to enhance resilience without extensively altering other characteristics.^[37] This method preserves breed standards by recovering 93-99.9% of the recurrent parent's genome after 3 or more backcross generations, minimizing disruptions to established performance traits like productivity or conformation in livestock. Additionally, backcrossing is cost-effective for high-value animals, such as dairy cattle or laboratory rodents, as it leverages existing elite lines rather than developing entirely new populations, reducing the need for extensive new infrastructure or testing.^[38] Despite these benefits, backcrossing presents notable challenges, especially in reproductive and welfare contexts unique to animals. Repeated matings to the recurrent parent can increase inbreeding coefficients, leading to inbreeding depression that manifests as reduced fertility, lower litter sizes, and higher neonatal mortality rates in species like cattle and mice.^[39] Ethical concerns arise from the potential stress imposed on animals through multiple breeding cycles, including physical strain from repeated pregnancies or artificial insemination, which can compromise welfare in confined livestock operations or laboratory settings.^[40] Progress is often slower in animals compared to plants due to longer generation intervals—typically 1-4 years in mammals—requiring extended timelines for genome recovery and trait stabilization.^[38] Regulatory hurdles further complicate applications involving genetically modified introgressions, such as those for enhanced disease resistance, due to stringent oversight on animal biotechnology to ensure food safety and environmental containment.^[41] In animal populations, backcrossing uniquely impacts population genetics by potentially eroding heterozygosity and overall diversity, particularly in endangered breeds where introgressed traits may inadvertently reduce adaptive variation and increase vulnerability to environmental changes.^[42] This loss of diversity is a greater concern in closed breeding systems common to purebred livestock or captive endangered species. Overall, backcrossing proves more suitable for controlled livestock production than wildlife conservation efforts, where environmental uncontrollability and ethical restrictions on captive hybridization limit its feasibility.^[43]

Specific Examples

In cattle breeding, backcrossing has been employed to introgress the polled (hornless) trait from Angus cattle into Holstein dairy lines, primarily during the 1990s and 2000s, to mitigate injury risks to animals and handlers associated with dehorning procedures.^[44] This process involves repeated backcrosses to Holstein sires while selecting for the dominant polled allele (Pc), achieving high genome recovery rates of over 90% after four to five generations, thereby preserving milk production traits while eliminating horns.^[44] The approach has resulted in polled Holstein populations that reduce welfare concerns and labor costs, with early programs demonstrating successful integration without significant loss in dairy performance.^[44] In poultry breeding, backcrossing of sex-linked genes, such as the barring gene (B) on the Z chromosome, has been utilized since the 1970s to develop auto-sexing broiler lines that enable visual sex identification at hatch based on down color differences between males and females.^[45] Commercial broiler programs cross non-barred broiler dams with barred sires from specialized lines, followed by backcrossing to recover broiler growth traits while fixing the sex-linked marker, allowing efficient culling of males in egg-laying operations or targeted rearing in meat production.^[45] This technique has improved hatchery efficiency by up to 20-30% in sexing accuracy, supporting large-scale broiler production without invasive methods.^[46] Backcrossing is also widely used in laboratory animals to create congenic strains, particularly in mice, where a specific genomic region from a donor strain is introgressed onto the genetic background of an inbred recurrent strain. This facilitates the study of gene functions, disease models, and histocompatibility by isolating the effects of the target locus while recovering over 99% of the recurrent genome after 10 or more backcrosses, often accelerated by marker-assisted selection.^[4] More recently, up to 2025, backcrossing of PRRS (porcine reproductive and respiratory syndrome) resistance-associated QTL, such as the major SNP on chromosome 4 (WUR10000125), into commercial pig lines has incorporated embryo transfer to accelerate introgression and enhance pandemic resilience against this economically devastating virus.^[47] Donor embryos from resistant commercial populations are transferred into elite recipient sows, followed by marker-assisted backcrossing to recover over 95% of the recipient genome in three to four generations, minimizing viremia and improving growth under PRRS challenge.^[48] This method has demonstrated up to 25% reduction in disease impact in field trials, supporting sustainable swine production amid ongoing outbreaks.^[49]

Advanced Techniques

Marker-Assisted Backcrossing

Marker-assisted backcrossing (MABC) is a molecular breeding technique that enhances traditional backcrossing by using genetic markers to select progeny carrying desired traits from a donor parent while recovering the genetic background of a recurrent elite parent.^[24]^[26] The process involves two main selection strategies: foreground selection, which targets markers tightly linked to the gene of interest (such as simple sequence repeat (SSR) or single nucleotide polymorphism (SNP) markers) to confirm the presence of the donor allele, and background selection, which employs markers distributed across the genome to maximize recovery of the recurrent parent's genome and minimize linkage drag from the donor.^[24]^[10] In foreground selection, markers flanking the target locus are used to identify and pyramid multiple genes if needed, particularly useful for recessive traits or those difficult to phenotype.^[24] Background selection complements this by screening for recurrent parent alleles at non-target loci, often requiring 50-100 markers for effective genome coverage in plants or animals with larger genomes.^[26] This dual approach allows breeders to accelerate the introgression process compared to conventional methods, which rely solely on phenotypic selection and typically require 6-8 generations for >95% recurrent genome recovery.^[24] MABC significantly improves efficiency by reducing the number of backcross generations needed to 3-4, enabling early achievement of over 95% recurrent genome recovery in selected progeny.^[24]^[26] For instance, simulations show that with optimal marker density and selection intensity, donor genome retention can be limited to 5-15% after three generations, far surpassing the theoretical 6.25% without markers.^[26] Key tools for MABC include quantitative trait locus (QTL) mapping to identify and validate linked markers for foreground selection, ensuring high specificity and reducing false positives.^[24] Software such as PLABSIM facilitates simulation of backcross designs, optimizing marker placement, population size, and selection strategies to predict outcomes and minimize generations.^[50] Since the 1990s, MABC has been widely adopted in both plant and animal breeding for introgressing traits like herbicide resistance, with notable applications including glyphosate tolerance in maize via SSR-linked markers and QTL regions in congenic mouse strains for disease modeling.^[24]^[26]

Integration with Modern Tools

Backcrossing integrates seamlessly with CRISPR-Cas9 gene editing to facilitate the precise transfer of edited alleles into elite germplasm backgrounds, minimizing linkage drag and off-target mutations associated with initial transformations. In this approach, CRISPR-Cas9 is first employed to introduce targeted modifications in a donor line, followed by repeated backcrossing to the recurrent elite parent to recover desirable agronomic traits while segregating away unintended edits or residual donor genome segments. This synergy enhances breeding efficiency by combining the specificity of gene editing with the recovery power of backcrossing.^[51] Synergizing backcrossing with genomic selection (GS) enables efficient improvement of polygenic traits by leveraging genomic estimated breeding values (GEBVs) to predict and select favorable allele combinations across complex backgrounds. During backcrossing, GS models, such as G-BLUP using genome-wide SNPs, forecast progeny performance for traits like yield and height, allowing early identification of lines recovering over 95% of the recurrent parent's genome while retaining polygenic gains from the donor. In oil palm backcross populations, this integration has achieved prediction accuracies up to 0.44 for yield-related traits, reducing the breeding cycle from approximately 20 years to 7 years compared to phenotypic selection alone and increasing the frequency of superior alleles in fewer generations. Such methods are particularly valuable for traits with low heritability, where traditional backcrossing alone struggles to capture additive genetic effects.^[52] Speed breeding protocols accelerate backcross timelines by manipulating environmental conditions to expedite generation advancement, often reducing the typical 6-12 month cycle to 2-3 months per generation. Utilizing LED lighting for extended photoperiods (up to 22 hours daily) and controlled temperatures (20-25°C), these systems promote rapid flowering and seed set without compromising fertility, while supplemental hormones like gibberellins can further hasten vegetative-to-reproductive transitions in certain species. In wheat and barley backcrossing programs, speed breeding has enabled up to six generations annually, facilitating faster introgression of traits such as drought tolerance into elite lines and integrating with marker-assisted selection for precise recovery. This approach has been pivotal in global breeding efforts, as outlined in foundational protocols that emphasize scalable growth chamber setups.^[53] As of November 2025, regulatory frameworks in jurisdictions like the United States continue to evolve regarding backcrossed gene-edited crops, with ongoing debates over their classification as non-GMO equivalents. Following the December 2024 federal court vacatur of the SECURE rule, USDA reverted to pre-2020 regulations and issued conforming amendments effective June 16, 2025, for the movement of certain genetically engineered organisms. Under these pre-SECURE exemptions, plants derived from CRISPR edits without integrated foreign DNA—often refined via backcrossing—are generally not subject to biotechnology oversight if they pose no greater plant pest risk than non-edited counterparts, allowing deregulation for commercial release. The Spring 2025 Unified Agenda, published in October 2025, includes proposed rules to address biotechnology products, reigniting discussions on process- versus product-based regulation. Advocates argue that backcrossed edited lines, indistinguishable from conventional varieties at the molecular level, should evade GMO labeling to foster innovation. This perspective aligns with FDA and EPA assessments confirming safety equivalence for approved cases, though legal uncertainties persist pending further rulemaking.^[54]^[55]^[56]^[57]

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Invited review: Breeding and ethical perspectives on genetically ...
In this review, we discuss applications of both classical genetic modifications (GM) using vectors and genome editing in dairy cattle breeding.
[39]
[PDF] Hybridization and “Genetic” Extinction
• Retaining genetic diversity among species increases evolutionary potential. • Hybridization compounds effects of loss of genetic diversity in small ...
[40]
Analysis: The thorny ethics of hybrid animals | PBS News
Oct 27, 2017 · Due to all these factors, hybridization in captivity is widely opposed by conservationists.
[41]
Assessment of genomic selection for introgression of polledness into ...
The main objective of the current study was to evaluate the use of genomic selection to speed up the introgression of a target allele in a conventional dairy ...
[42]
US6130365A - Breeding lines of color-sexable day-old chicks and ...
To establish a female parent line for producing auto-sexed broiler chicks, a female line pure for the sex-linked silver gene, S, must be established. For easy ...
[43]
[PDF] Sexing of Poultry - UC ANR Portal
This method, known as autosexing, involves the inter- action of a sex-linked gene-the barring gene in every instance reported to date- with certain other genes ...Missing: backcrossing 1970s<|separator|>
[44]
A Mutation in the Myostatin Gene Increases Muscle Mass and ... - NIH
May 25, 2007 · Owners report that “bully” whippets do not have any health abnormalities other than muscle cramping in the shoulder and thigh. However, the dogs ...
[45]
Genetic resistance - an alternative for controlling PRRS? - PMC
Nov 16, 2016 · More than 2,500 quantitative trait loci (QTL) have been published for health parameters in the pig, among them 400 for resistance/susceptibility ...
[46]
The WUR0000125 PRRS resilience SNP had no apparent effect on ...
Jul 24, 2023 · Many studies have shown that there is substantial genetic variation in the resistance, tolerance, and resilience of pigs to PRRSV infection, and ...Missing: introgression | Show results with:introgression
[47]
https://pmc.ncbi.nlm.nih.gov/articles/PMC5382513/
[48]
Computer note. PLABSIM: software for simulation of marker-assisted ...
PLABSIM simulates marker-assisted selection in backcross programs with arbitrary genetic design. It evaluates gene and genotype frequencies, homozygosity, and ...
[49]
CRISPR/Cas9-mediated editing of eukaryotic elongation factor 1B ...
Feb 22, 2025 · CRISPR/Cas9-mediated editing of eukaryotic elongation factor 1B gamma ... backcrossing to introgress desired traits into elite backgrounds.
[50]
Genomic selection for morphological and yield-related traits using ...
Nov 18, 2022 · It has been proposed that combining backcrossing with genomic selection could increase the frequency of favorable alleles in a shorter ...
[51]
Speed breeding is a powerful tool to accelerate crop research and ...
Jan 1, 2018 · We present a method called 'speed breeding', which greatly shortens generation time and accelerates breeding and research programmes.Missing: backcrossing | Show results with:backcrossing
[52]
Revised Biotechnology Regulations (Previously SECURE Rule)
The revised rule differs from the previous regulatory framework by focusing on an organism's properties and not on the method used to produce it, enabling APHIS ...Missing: backcrossed | Show results with:backcrossed
[53]
Federal Judge Vacates USDA Rule Regulating Biotech Crops
Jan 6, 2025 · On December 2, 2024, the United States District Court of the Northern District of California vacated the US Department of Agriculture (USDA) SECURE Rule.Missing: backcrossed | Show results with:backcrossed