Backcross in Genetics and Plant Breeding

What is back cross? Backcrossing is the method of mating a hybrid with one of its parents or a genetically identical individual in an attempt to develop infants with a genetic identity identical to that of the parent. Animal breeding, horticulture, and the development of gene knockout species all use it. Backcrossed hybrids are often referred to as "BC" hybrids. For instance, a BC1 hybrid is an F1 hybrid mated with one of its parents (or a genetically identical individual), and a BC2 hybrid is a BC1 hybrid mixed with the same parent (or a genetically identical individual).

Backcross can be defined as the method of crossing or mating a hybrid offspring with one of its parents or to a genetically identical individual in a trial to create infants which are genetically identical to the parent. It is useful and used in the fields of horticulture, animal breeding, and even in the development of knockout of genes in species. 

The hybrids that are backcrossed are called the ‘BC’ hybrids. For example, a BC1 hybrid is an F1 hybrid that was crossed with one of its parents or a genetically identical species. The BC2 hybrid can be defined as a BC1 hybrid that is mixed with the same parent or the genetically identical species. Other examples are the backcrossing that is done in the animals. Animals that have a recessive or poor genetic trait can be backcrossed with a species with a good/beneficial or dominant trait so that a species with a better genetic trait is created. The knockout species is backcrossed against the species which has the required genes during the knockout experiments. It is conveniently carried out to culture stem cell lines which is required for a species’ required genes. 

Backcrossing in Animals

Backcrossing in Animals is one of the back cross examples. Animals can be backcrossed to shift a beneficial trait from an animal with a poor genetic background to an animal with a better genetic background. The knockout animal has been backcrossed against the animal with the necessary genetic background during gene knockout experiments in specific, where the knockout is conducted on conveniently cultured stem cell lines but which is necessary for an animal having a distinct genetic background.

When a mouse possessing the desirable traits (in such case, the absence of a gene, for instance, a knockout, as shown by the occurrence of a positive selectable marker) is mated with a mouse with constant genetic background, the mean percentage of the offspring's genetic material taken from that constant background rises. After enough iterations, the outcome is an animal mostly with the desired phenotype in the desired genetic history, having the amount of genetic material from the initial stem cells decreased to the bare minimum (on the scale of 0.01 percent).

The proportion of genetic material originating within each cell line will differ amongst offspring of a particular crossing based on the nature of meiosis, wherein the chromosomes originating from each parent are spontaneously shuffled and allocated to each nascent gamete but it will have an estimated value. Each offspring's genotype can be analyzed to select not only a person with the preferred genetic trait but also one with the least amount of genetic material from the actual stem cell line.

An inbred strain carrying one of its chromosomes substituted by the homologous chromosome of some other inbred strain through a sequence of marker-assisted backcrosses is known as a consomic strain.

Purpose of Back Cross

According to the back cross definition, the Purpose of the back cross is: 

• It is useful for isolating (separating out) unique characteristics in a similar group of animals or plants.

• Since the new cultivar can be adapted to the same area as the original cultivar, the approach decreases the amount of field testing necessary.

• Backcross breeding can be done over and over again. The same backcrossed cultivar can be recovered if the same parents are used.

• It's a traditional approach that prevents new recombination.

• It can be used to insert unique genes into massive crosses.

• It can be used to insert unique genes into massive crosses.

• It can be used to breed self-pollinated and cross-pollinated plants.

Test Cross

In genetics, a person expressing a dominant phenotype could have two copies of the dominant allele (homozygous dominant) or one copy of each dominant and recessive allele (heterozygous dominant). A test cross may be used to decide whether a person is homozygous dominant or heterozygous dominant.

In a test cross, the person in question is bred with another homozygous for the recessive gene, and the test cross's offspring are tested. Since a homozygous recessive organism may only pass on recessive alleles, the offspring's phenotype is determined by the allele passed on.

Applications of Testcross in Model Organisms

There are many applications for test crosses. Caenorhabditis elegans and Drosophila melanogaster are two common animal species that are frequently used for test crosses. The following are the basic test cross procedures for these organisms:

C. Elegans 

Place worms of a known recessive genotype on an agar plate with worms of an unknown genotype to perform a test cross with C. elegans. Give time for the hermaphrodite and male worms to mate and reproduce. The dominant parent's genotype can be determined by looking at the ratio of recessive to dominant phenotypes under a microscope.

D. Melanogaster

Select an allele with a known dominant and recessive phenotype for a test cross with D. melanogaster. The dominant eye color is red, while the recessive eye color is white. Fill a single tube with virgin females with white eyes and young males with red eyes. Remove parental lines until offspring appear as larvae and study the phenotype of adult offspring.

Limitations

Testing crosses has a number of drawbacks. It can be a lengthy process, as certain species need a long period of growth in each generation to achieve the desired phenotype. Statistics necessitate a large number of descendants in order to obtain accurate results. If dominance is complete, test crosses are only useful. When the dominant and recessive alleles overlap in the offspring, the result is a combination of the two phenotypes. 

When a single allele generates a number of phenotypes, which isn't accounted for in a research cross, the term variable expressivity is used. The test cross is becoming less common in genetics as more sophisticated techniques for determining genotype arise. Genetic testing and genome mapping are modern advancements that allow for more effective and accurate genotype details to be determined. Test crosses, on the other hand, are still used today and have laid a solid basis for the advancement of more advanced techniques.