page_title: Backcrossing
section_title: Animals
hierarchical_section_title: Backcrossing / Animals
context_page_description: 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 organisms.
  Backcrossed hybrids are sometimes described with acronym "BC", for example, an F1 hybrid crossed with one of its parents can be termed a BC1 hybrid, and a further cross of the BC1 hybrid to the same parent produces a BC2 hybrid.
context_section_description: 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 desired genetic background, with the percentage of genetic material from the original stem cells reduced to a minimum (in the order of 0.01%).
  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 an expected value. 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.
  
  A consomic strain is an inbred strain with one of its chromosomes replaced by the homologous chromosome of another inbred strain via a series of marker-assisted backcrosses.
caption: 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.