- Backcrossing is a breeding system in which a crossbred or hybrid offspring is repeatedly mated back to one of its parental breeds, lines, strains, or populations. The main purpose is usually to increase the genetic contribution of a desired parent population while retaining some genetic material from the other parent. In animal breeding, backcrossing can be used to introduce specific desirable characteristics, improve production performance, recover a preferred breed type, or combine genetic advantages from different populations. It is closely related to crossbreeding, grading up, breed composition, and genetic improvement, but has a more specific mating structure in which descendants are repeatedly crossed with one parental type.
- A typical backcross begins with a cross between two populations, such as breeds A and B. The first-generation offspring contains approximately 50% genetic contribution from each breed. If the offspring is then mated to breed A, the expected genetic composition of the next generation becomes 75% A and 25% B. Repeated mating to breed A produces approximately 87.5% A and 12.5% B, followed by 93.75% A and 6.25% B, and so on. The expected contribution of the recurrent parent after n generations of backcrossing can be expressed as:
- P_n = 1 – (1/2)^n
- where P_n is the expected proportion contributed by the recurrent parent after n backcross generations, assuming the starting cross contains 50% genetic contribution from that parent.
- Backcrossing can therefore be used to progressively increase the contribution of a preferred parental breed while retaining a smaller proportion of genes from another breed. However, the actual genetic composition of individual animals will vary because of Mendelian segregation and Mendelian sampling. The expected breed proportion describes the population average rather than the exact genome of every individual animal.
- One important application of backcrossing is the transfer of a desirable genetic characteristic from one population into another. For example, a breed with excellent production performance may be crossed with another population carrying valuable disease resistance, adaptation, heat tolerance, or another desirable trait. Subsequent backcrossing to the high-performing breed can progressively restore its genetic background while retaining the desired characteristic, particularly when selection is applied to identify animals carrying the target genetic variation.
- Backcrossing can also be used to improve breed composition. If a population contains undesirable or less suitable characteristics but possesses a small amount of valuable genetic material, repeated backcrossing to a superior breed can shift the population toward the desired breed type. This makes backcrossing closely related to grading up, although the two concepts are not always identical in purpose. Grading up generally refers to a longer-term strategy for progressively increasing the contribution of a selected superior breed in an existing population, whereas backcrossing more specifically describes repeated mating of descendants to one parental type.
- The choice of the recurrent parent is therefore extremely important. The recurrent breed should possess the characteristics that the breeding program intends to retain or recover, while the donor population provides the genetic contribution that is being introduced. Selection within each generation can increase the probability that the desired genetic characteristics are retained while unwanted characteristics are removed.
- Backcrossing can be performed using conventional phenotypic selection, family selection, pedigree information, progeny testing, breeding values, estimated breeding values (EBVs), or genomic selection. Genomic information can be particularly useful because animals can be selected according to their genome-wide similarity to the recurrent parent while simultaneously retaining or identifying desirable genes or genomic regions from the donor population. This can make modern backcrossing more precise than relying only on phenotype and pedigree.
- The expected breeding value of an offspring from two parents can be represented simply as: E(A_offspring) = (A_sire + A_dam) / 2
- where A_sire and A_dam are the breeding values of the parents. However, the realized genetic merit of an individual also depends on Mendelian sampling, so offspring from the same parents are not genetically identical. This is why selection among backcross offspring remains important even when the parental breeds are known.
- Backcrossing can influence heterosis differently from structured crossbreeding systems. The first cross between genetically different breeds may show substantial heterosis, but repeated backcrossing generally reduces the proportion of the genome originating from the donor breed and consequently reduces the level of breed-derived heterozygosity relative to the original cross. If maintaining heterosis is a major objective, a rotational crossbreeding system, terminal crossbreeding system, or another structured crossbreeding strategy may be more appropriate.
- The success of backcrossing also depends on genetic correlations, genotype–environment interaction (G×E), and the biological relationship between the traits being selected. A donor breed may provide valuable genetics for one trait but may also introduce unfavorable characteristics for another. Therefore, backcrossing should normally be incorporated into a broader breeding objective rather than focusing on a single characteristic without considering overall genetic merit.
- Maternal effects can also be important in backcrossing programs. The breed of the dam influences the prenatal environment, milk production, maternal behaviour, and early-life performance of offspring. Consequently, the genetic composition of the offspring should not be interpreted independently of the maternal environment in which it develops.
- Backcrossing can also affect genetic diversity and inbreeding. Repeated use of a limited number of sires from the recurrent breed can increase genetic concentration and reduce the effective population size. The approximate relationship between effective population size and the rate of inbreeding is: ΔF ≈ 1 / (2Ne)
- where ΔF is the approximate increase in inbreeding per generation and Ne is the effective population size. Maintaining an adequate number of unrelated or distantly related breeding animals is therefore important when backcrossing is used over several generations.
- Genomic relatedness, pedigree-based relatedness, and runs of homozygosity (ROH) can help monitor changes in genetic diversity during a backcrossing program. Selection should avoid unnecessarily increasing relatedness among breeding animals, particularly when the program is conducted within a relatively small population.
- Backcrossing may also be useful for transferring genetic adaptation from one population to another. For example, locally adapted animals may possess valuable genetic characteristics for heat tolerance, disease resistance, feed utilization, or survival under challenging environmental conditions. If these characteristics are combined with the production potential of another breed, backcrossing can be used to recover much of the desired production background while attempting to retain important adaptive genetic variation.
- The economic value of backcrossing depends on the breeding objective, production environment, generation interval, selection accuracy, reproductive efficiency, and value of the introduced traits. A theoretically desirable genetic change may not provide an economic advantage if it increases management costs, reduces fertility, or negatively affects other economically important traits. For this reason, economic weights, selection indexes, and balanced breeding goals can be incorporated into backcrossing programs.
- Modern backcrossing can be strengthened by combining conventional selection with genomic selection, reproductive technologies, accurate performance recording, and genetic evaluation. Genomic information can help identify animals that have the desired genetic contribution from the donor population while having a high proportion of the preferred recurrent genetic background. This approach is sometimes described as genomic-assisted backcrossing and can accelerate the recovery of the desired genetic background.
- Backcrossing is different from purebreeding, because purebreeding normally involves mating animals within the same breed without deliberately introducing another breed in the preceding generations. It is also different from outcrossing, which generally involves mating relatively unrelated animals within the same breed or population. Compared with crossbreeding, backcrossing has a specific objective of repeatedly returning offspring to one parental population. Compared with grading up, backcrossing emphasizes repeated mating to a parental type, while grading up is generally used to progressively increase the contribution of a selected breed across generations.
- The main advantages of backcrossing include the ability to introduce valuable genetic characteristics, progressively recover a preferred breed background, combine complementary genetic resources, and provide a structured method for genetic improvement. Its limitations include loss of heterosis over generations, possible introduction of unfavorable genes, increased genetic concentration, management complexity, and the time required to obtain a population with the desired genetic composition.
- A successful backcrossing program therefore requires clearly defined breeding objectives, appropriate parental breeds, accurate selection criteria, reliable genetic evaluation, careful monitoring of breed composition, and management of genetic diversity. When appropriately designed, backcrossing can be a useful tool for combining the strengths of different populations while progressively recovering the genetic background of a preferred breed. Its greatest value is achieved when it is integrated with quantitative genetics, genomic selection, genetic evaluation, breed conservation, and sustainable genetic improvement.
