Backcross
What is a backcross in genetics?
What is the purpose of backcrossing in plant breeding?
What is the difference between foreground and background selection in backcrossing?
How is a recessive gene backcrossed differently from a dominant gene?
Backcross, the mating of a hybrid organism (offspring of genetically unlike parents) with one of its parents or with an organism genetically similar to the parent. The backcross is useful in genetics studies for isolating (separating out) and transferring certain characteristics in a related group of animals or plants.
In animal breeding, topcrossing and grading are forms of backcrossing. Grading usually refers to the mating of average, or āgrade,ā females to a superior male and then backcrossing the female offspring to the same or a similar sire. In plants, backcrossing is used to introduce desirable traitsāsuch as disease and pest resistance, salt and drought tolerance, and improved yield qualitiesāfrom an agronomically poor plant to a cultivar with an otherwise superior genetic background. Although backcrossing is used in both animals and plants, biological and logistical limitations associated with animals make backcrossing a less efficient and less commonly applied method compared with its use in plant breeding.
Backcross breeding in plants
The agronomically superior plant that receives the gene controlling the desired trait (target gene) is called the recurrent parent, because it is crossed multiple times during backcrossing. The donor of the desirable trait is called the nonrecurrent parent, because it is used only once during the procedure. In the resulting line, it is possible to recover about 98 percent of the genetic composition of the recurrent parent, along with the desired trait from the donor parent.
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The target gene in the donor plant that controls the desirable trait may be the product of either mutation or genetic engineering. Most such genes are inherited as dominant alleles (gene variants), although some may be recessive. (See also āWhatās the Difference Between a Gene and an Allele?ā)
Backcrossing of a dominant gene
The simplest backcross involves the transfer of a single dominant allele. Pollen from the inbred donor plant, which is homozygous for the target gene (meaning that it contains two dominant alleles) is transferred to the inbred recurrent parent that is homozygous for the recessive allele. The resulting progeny (designated F1, for first filial generation) are uniformly heterozygous for the target gene (meaning that they contain one dominant and one recessive allele). These plants carry 50 percent of the genome (complete set of genetic material) from the donor and 50 percent from the recurrent parent.
When the desired gene is recessive, breeders usually perform repeated backcrossing along with self-pollination and progeny testing to find plants that carry the recessive allele. Molecular genetic tests are commonly used to identify these plants more quickly, reducing the time needed to develop the new line.
Molecular markers located close to the recessive gene help breeders easily identify heterozygous plants without self-pollination. This method is called foreground selection. Breeders can also use markers to select plants that carry the recurrent parentās genes at all locations except the target gene. This helps remove unwanted genes inherited from the donor parent. This process, known as background selection, allows breeders to maintain control over the genetic background of the improved line.
In the next growing season, the F1 plants are crossed with the recurrent parent, producing equal proportions of homozygous recessive and heterozygous progeny for the target gene. Each time the hybrid plant is crossed with the recurrent parent, the amount of donor genome is reduced by half. As a result, the progeny, designated BC1F1 for an F1 hybrid derived from the first backcross, contain 75 percent of the recurrent parent genome and 25 percent of the donor genome. Only the heterozygous BC1F1 plants are used for further backcrossing.
The resulting BC2F1 progeny contain 87.5 percent of the recurrent parent genome and 12.5 percent of the donor genome. Additional backcrosses further reduce the proportion of the donor genome, until the resulting line contains about 98 percent of the recurrent parent genome while retaining the dominant allele of the target gene. The dominant allele of a target gene is maintained in the heterozygous state during backcrossing because only the heterozygous progeny of each backcross carry this allele. After the final backcross, the heterozygous progeny may be self-pollinated to produce plants that are homozygous for the desired gene.
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| generation** | recurrent parent genome (%) | donor parent genome (%) |
|---|---|---|
| *Absence of linkage between the target gene and other genes is assumed. | ||
| **F is filial generation and BC is backcross. Subscript numbers represent the sequence of the crossing event. BC1 is backcross event 1, and BC1F1 is the first filial generation derived from the first backcross, and so on. | ||
| F1 | 50 | 50 |
| BC1F1 | 75 | 25 |
| BC2F1 | 87.5 | 12.5 |
| BC3F1 | 93.75 | 6.25 |
| BC4F1 | 96.875 | 3.125 |
| BC5F1 | 98.4375 | 1.5625 |