Backcross

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In flowering plants, backcrossing occurs through pollination of the female flower organ and fertilization of the egg cell located therein with the pollen of a plant from the generation of its offspring or through pollination and fertilization of a daughter plant with the pollen of one of the two plants of the parent generation .

Gregor Mendel used this method for the first time in a targeted manner , firstly as a test method to check the genotype of plants for purity and secondly in order to be able to use purely homozygous parent plants for a certain trait for his genetic experiments. Through his well-thought-out approach, he succeeded in discovering the Mendelian rules .

In animals , backcrossing takes place by mating back a female animal with its father or by mating a male animal back with its mother. Both occur regularly in nature and are occasionally used in animal breeding . Crossbreeding experiments in animals have shown that most of the characteristics are inherited according to Mendel's rules in animals too .

When dominant traits are inherited, it is not possible to tell from the phenotype (appearance) of the individual individuals which gene combination they have inherited. Because there are two alleles for each trait with simple Mendelian inheritance (the one from the father's side and the one from the maternal side), but the recessive allele is not expressed, it is not possible to determine in individuals with a dominant characteristic in the phenotype whether the genetic makeup is homozygous. or mixed (heterozygous).

The homozygous carriers of a recessive trait, on the other hand, can easily be recognized by their appearance, because recessive hereditary factors are only phenotypically expressed in these. This means that if an individual shows a recessive characteristic in the phenotype, one can assume the purity of this genetic make-up.

A cross with a recessive homozygous parent can serve as a test procedure to check carriers of dominant traits for purity or mixed inheritance, for example in individuals of the F2 generation ( split rule ). If there is a further breakdown into both phenotypes, the bearer of the dominant trait was mixed.

The term backcrossing has a second meaning in the mating of cultivated plants or domestic animals with specimens of their wild form , which is usually intended to refresh the gene pool and restore certain vitality characteristics.

Gregor Mendel used pea plants for his genetic test series .

Example: flower color of pea plants

Backcrosses of red flowering plants of the F2 generation

In pea plants there is one allele for red (= B) and one for white (= b) flower color with regard to flower color. The allele B for the formation of the red dye is dominant over the allele b, which causes the lack of the dye.

In a red-flowering pea plant, the genotype BB (homozygous, homozygous) or Bb (mixed, heterozygous) can be present.

In order to determine this, one can (in addition to the DNA analysis possible today ) use a backcross as a test procedure. Here, the to be examined individual with a homozygous recessive specimen of this species crossed , in this case with a white flowering pea plant. Verification may be required because 25 percent of individuals in the F2 generation carry the dominant allele homozygous according to the cleavage rule , while 50 percent are heterozygous.

If the red-flowering plant is pure breeding, you will find 100% red-flowering offspring in the next generation. If the red-flowered plant is mixed, you will get 50 percent red-flowered and 50 percent white-flowered pea plants in the next generation.

Backcrossing to examine traits inherited as intermediates is unnecessary, as the genotype can be read off from the phenotype.

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