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What is Back Cross and Test Cross?

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.


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.

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FAQs on Backcross

1. Give the Significance of the Test Cross.

The test cross is used to assess whether a dominant parent is heterozygous or homozygous for its genotype. It is possible to predict which genotype the parent has based on the outcome produced in the ratio of the offspring.

2. Specify the Major Difference Between the Back Cross and the Test Cross.

The major difference between test cross and back cross is The test cross assists in the detection of the dominant individual's genotype whereas By examining gene segregation during gamete formation, the back cross aids in the detection of the elite genotype.

3. What is a test cross?

In genetics, species can express a dominant phenotype which could have two copies of the dominant allele which is called the homozygous dominant. Another phenotype is when it will have both a copy of the dominant allele and a recessive allele which is called the heterozygous dominant. The test cross can help us decide whether the species is homozygous dominant or whether it is heterozygous dominant. In such a cross, the species is mated with another homozygous for the recessive gene and then the offspring of the test cross is tested. A homozygous recessive organism may transfer recessive alleles and thus, the offspring’s phenotype is determined by the allele that is passed or transferred to it. 

4. What are the disadvantages of a backcross?

The disadvantages or demerits of a backcross can be defined as:

  • Backcross can act very poorly for quantitative traits

  • It is more restricted towards recessive traits

  • In reality, some of the parts of the genome from the parents are still present and give some uncommon traits that are associated with them

  • For larger backcrosses, recombination may have many ‘alien’ genes

  • Backcrosses are required to produce a new cultivar which can take years. 

5. What are the uses of a backcross?

Uses of the back cross can be defined as:

  • Backcrosses are used for the isolation or separation of unique genes in a similar population of species.

  • The cultivar can be adapted to the same area as that of the original, the backcross decreases the amount required for field testing

  • Backcross can be done repeatedly and the species which were previously backcrossed can be backcrossed again. 

  • It can prevent new recombination and is considered a  traditional approach.

  • This can be used for the insertion of unique and beneficial genes in the crosses

  • It is useful in the breeding of cross-pollinated and self-pollinated plants.

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