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Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene knockout animals.

Plants

Advantages

• If the recurrent parent is an elite genotype, at the end of the backcrossing programme an elite genotype is recovered
• As there is no "new" recombination, the elite combination is not lost

Disadvantages

• Works poorly for quantitative traits
• Is more restricted for recessive traits
• In practice, sections of genome from the non-recurrent parents are often still present and can have deleterious traits associated with them
• For very wide crosses, limited recombination may maintain thousands of ‘alien’ genes within the elite cultivar

Natural backcrossings

York radiate groundsel (Senecio eboracensis) is a naturally occurring hybrid species of Oxford ragwort (Senecio squalidus) and common groundsel (Senecio vulgaris). It is thought to have arisen from a backcrossing of the F1 hybrid with S. vulgaris.¹

Animals

This image demonstrates backcrossing of a heterozygous mouse from one genetic background onto another genetic background. In this example, the gene knockout is performed on 129/Sv cells and then backcrossed into the C57B/6J genetic background. With each successive backcross, there is an increase in the percentage of C57B/6J DNA that constitutes the genome of the offspring.

Backcrossing may be deliberately employed in animals to transfer a desirable trait in an animal of inferior genetic background to an animal of preferable genetic background. In gene knockout experiments in particular, where the knockout is performed on easily cultured stem cell lines, but is required in an animal with a different genetic background, the knockout animal is backcrossed against the animal of the required genetic background. As the figure shows, each time that the mouse with the desired trait (in this case the lack of a gene (i.e. a knockout), indicated by the presence of a positive selectable marker) is crossed with a mouse of a constant genetic background, the average percentage of the genetic material of the offspring that is derived from that constant background increases. The result, after sufficient reiterations, is an animal with the desired trait in the desire genetic background, with the percentage of genetic material from the original stem cells reduced to a minimum (in the order of 0.01%).²

Due to the nature of meiosis, in which chromosomes derived from each parent are randomly shuffled and assigned to each nascent gamete, the percentage of genetic material deriving from either cell-line will vary between offspring of a single crossing but will have a predictable mean average. The genotype of each member of offspring may be assessed to choose not only an individual that carries the desired genetic trait, but also the minimum percentage of genetic material from the original stem cell line.³

See also

• Introgression

References

1. ^ Abbot, R.J.; Lowe, A.J. (2003). "A new British species, Senecio eboracensis (Asteraceae), another hybrid derivative of S. vulgaris L. and S. squalidae L" (PDF). Watsonia 24: pp. 375–388, http://www.watsonia.org.uk/Vol24p375.pdf. Retrieved on 15 July 2007. 
2. ^ "Embryonic Stem Cell". Retrieved on 2008-01-01.
3. ^ Frisch M, Melchinger AE (2005). "Selection theory for marker-assisted backcrossing". Genetics 170 (2): 909–17. doi:10.1534/genetics.104.035451. PMID 15802512, http://www.genetics.org/cgi/pmidlookup?view=long&pmid=15802512. 

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