Backcrossing is a crossing of a hybrid with one of its
parentA parent is a mother or father; one who sires or gives birth to and/or nurtures and raises an offspring. The different roles of parents vary throughout the tree of life, and are especially complex in human culture.- Father :...
s 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 knockoutA gene knockout is a genetic technique in which an organism is engineered to carry genes that have been made inoperative . Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function...
organisms.
Advantages
- If the recurrent parent is an elite genotype
The genotype is the genetic constitution of a cell, an organism, or an individual usually with reference to a specific character under consideration...
, at the end of the backcrossing programme an elite genotype is recovered
- As there is no "new" recombination
Recombination may refer to:* Genetic recombination, the process by which genetic material is broken and joined to other genetic material* Carrier generation and recombination, processes by which mobile electrons and electron holes are created and eliminated...
, the elite combination is not lost
Disadvantages
- Works poorly for quantitative traits
- Is more restricted for recessive traits
- In practice, sections of genome
In modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...
from the non-recurrent parents are often still present and can have unwanted traits associated with them
- For very wide crosses, limited recombination may maintain thousands of ‘alien’ genes within the elite cultivar
A cultivar is a cultivated plant that has been selected and given a unique name because of desired characteristics; it is usually distinct from similar plants and when propagated it retains those characteristics....
- Many backcrosses are required to produce a new cultivar which can take many years
Natural backcrossings
York radiate groundsel (
Senecio eboracensisSenecio eboracensis, the York groundsel or York radiate groundsel, is a self-pollinating hybrid species of ragwortand one of only six new plants to be discovered in either the United Kingdom or North America in the last 100 years....
) is a naturally occurring hybrid species of Oxford ragwort (
Senecio squalidusOxford Ragwort , a Ragwort from the daisy family Asteraceae, yellow-flowered herbaceous plant, native to mountainous, rocky or volcanic areas, has managed to find other homes on man-made and natural piles of rocks, war-ruined neighborhoods and even on stone walls that resemble their well drained...
) and common groundsel (
Senecio vulgaris). It is thought to have arisen from a backcrossing of the F1 hybrid with
S. vulgaris.
Animals
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
culturedCell culture is the process by which cells are grown under controlled conditions. In practice the term "cell culture" has come to refer to the culturing of cells derived from multicellular eukaryotes, especially animal cells...
stem cellStem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
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%).
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 valueIn probability theory and statistics, the expected value of a random variable is the integral of the random variable with respect to its probability measure....
. The
genotypeThe genotype is the genetic constitution of a cell, an organism, or an individual usually with reference to a specific character under consideration...
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.