A crossing of a creature with its parents to find out which allele of a gene it has. This is a technique that can be used to purify one strain by only keeping the desired genetic property.

If a gene has two alleles, G and g, and G is dominant over the g, then the two genotypes GG (homozygous) and Gg (heterozygous) will both come out looking the same: they will have the phenotype of the dominant G. To find out which they have, you mate them with creatures of known genotype gg.

If your test organism is Gg then a proportion of their offspring will inherit the recessive g from both parents and appear as phenotypically gg.

The dominance may be so clear-cut that a single backcross can decide it, e.g. normal coat colour versus albino. Here there are no intermediates. But with most genes the presence of the gene being sought is not so clear cut, so several generations of backcrossing may be used to increase the probability that the strain now contains the pure allele. A genotype GG mated with gg never produces gg offspring, but this might also happen by chance if its genotype was Gg: repeated backcrossing reduces the possibility of this.

A backcross is also known as a testcross. To get the same genetic effect, the mating does not actually have to be with a parent: e.g. any albino clearly has a known genotype, and would work as well.

Another common use for a backcross is to combine two quantitative traits at a given ratio. For example, beef cattle hardy enough to survive the climate of North Australia are bred by crossing Australian cows with Brahma bulls from India. The F1 progeny, which share 50% of their genotype with either parent, are then backcrossed to the Australian parents to achieve a 75% expression of the Australian genotype and 25% expression of the Indian -- enough to give the F2 offspring resistance to north Australian conditions. This is a useful way to introduce a novel gene into a population while retaining most of the original population's genotype; the disadvantage is that excessive backcrosses can lead to inbreeding depression. Successive backcrosses can be performed with different combinations of males and females to achieve varying degrees of heterosis and relatedness (as in a three-breed cross or rotational cross).

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