Difference Between Test Cross and Backcross

Main Difference – Test Cross vs Backcross

Test cross and backcross are two types of crosses introduced by Gregor Mendel. In test cross, a dominant phenotype is crossed with the homologous recessive genotype in order to discriminate between homologous dominant and heterozygous genotypes. In backcross, the F1 is crossed with one of the parents or genetically identical individual to the parent. The main difference between test cross and the backcross is that test cross is used to discriminate the genotype of an individual which is phenotypically dominant whereas a backcross is used to recover an elite genotype from a parent which bears an elite genotype.

This article explores, 

1. What is Test Cross
     – Definition, Function, Process
2. What is Backcross
     – Definition, Function, Process
3. What is the difference between Test Cross and BackcrossDifference Between Test Cross and Backcross - Comparison Summary

What is a Test Cross

The breeding of a dominant phenotype with the recessive phenotype is referred to as a test cross. Zygosity of the dominant phenotype can be identified by test cross. Zygosity is the degree of similarity between two alleles which determine a particular trait. The zygosity is identified by the proportion of phenotypes occurring in the offspring. It can be either homozygous or heterozygous. Homozygous individuals consist of either two dominant alleles or two recessive alleles. The heterozygous individuals contain both dominant and recessive alleles of the gene. 

If an individual exhibits the dominant phenotype, the genotype of that particular individual would be either homozygous dominant or heterozygous. In this situation, the exact genotype can be determined by performing a test cross with an individual exhibiting the recessive phenotype for that trait. The genotype of the recessive phenotype is always homozygous recessive for that particular trait. Therefore, the proportion of the phenotypes in the offspring may describe the zygosity of the dominant phenotype which is examined during the test cross.

Main Difference - Test Cross vs Backcross

Figure 1: Punnett Square Test Cross

The above diagram illustrates the Punnett square of the test cross performed for the pod colour of a pea plant. Dominant allele for the pod colour is dominated by Y whereas the recessive is dominated by y. Here, yellow is the dominant colour of the pod whereas green is the recessive pod colour. The allele combination of the homologous dominant is YY, exhibiting the yellow colour pods. Yy is the allele combination of heterozygous, exhibiting the yellow colour pods. The allele combination of the homozygous recessive is yy, exhibiting the green colour pods.

The genotype of the pea which exhibits the yellow pod colour can be either YY or Yy. The discrimination between YY and Yy can be achieved by mating that particular pea with a pea exhibiting the green colour pods (yy). If the genotype of the yellow colour pod is Yy, the offspring consists of 50% yellow colour pods and 50% green colour pods as in the first Punnett square in the above figure. On the other hand, if the genotype is YY, the offspring consists of only the yellow colour pods. Hence, the genotype of the dominant phenotype can be identified depending on the colour of the pods resulting in the offspring.

What is a Backcross

The breeding of F1 hybrid with one of the two parents is referred to as a backcross. When F1 is bred with the homozygous dominant, the offspring produces a 100% dominant phenotype. When the F1 is bred with a recessive phenotype, the offspring produces 50% dominant and 50% recessive phenotypes. This cross produces an offspring which is genetically identical or closer to the parents of the F1. Hence, backcross is often used in horticulture and animal breeding in order to achieve genetically identical offspring carrying elite genotypes.

The offspring of the F1 hybrid, backcrossed with the recurrent parent is referred to as BC1 hybrid. The offspring of the BC1 hybrid, backcrossed with the recurrent parent is referred to as BC2 hybrid. By a backcross, an elite genotype can be recovered if the recurrent parent consists of an elite genotype.

Natural backcross can be observed in yolk radiate groundsel, arising from F1 hybrid backcross with the common groundsel. During the backcross, other useful traits can be diluted. In order to overcome this issue, the hybrids are repeatedly backcrossed with the recurrent parent. This may accumulate the useful traits in BC hybrids.     

Difference Between Test Cross and Backcross

Figure 2: Backcrossing Mouse

Difference Between Test Cross and Backcross

Definition

Test cross: Test cross is the breeding of dominant phenotype with its recessive phenotype.

Backcross: Backcross is the breeding of F1 hybrid with one of the parents.

Classification

Test cross: All test crosses are backcrosses.

Backcross: Backcross of F1 hybrid with the recessive phenotype can be considered as a test cross.

Breeding

Test cross: The F1 hybrid is crossed with recessive genotype in test cross.

Backcross: The F1 hybrid is crossed with either homozygous dominant or heterozygous genotypes in backcross. 

Function

Test cross: Test cross identifies the zygosity of the dominant phenotype.

Backcross: Backcross recovers the elite genotype.

Conclusion

During backcross, an individual is bred with its parent or with an individual genetically identical to the parent. This parent would be either homozygous dominant, heterozygous or homozygous recessive. By performing a test cross, an elite genotype can be recovered. In the test cross, the dominant phenotype is bred with the recessive phenotype. Hence, each test cross is a kind of backcross. By performing a test cross, the zygosity of the dominant phenotype can be identified. Therefore, the key difference between test cross and backcross is with their role of action.

Reference:
1. “Test cross”. Wikipedia, the free encyclopedia, 2016. / Accessed 18 Feb. 2017
2. “Backcrossing”. Wikipedia, the free encyclopedia, 2016. / Accessed 18 Feb. 2017
3. Robbins m. “Backcrossing, Backcross (BC) Populations, and Backcross Breeding”. Extension, 2012. / Accessed 18 Feb. 2017

Image Courtesy:
1. “Punnett Square Test Cross.PNG”. By KatieAnn127 – Own work Via
2. “Backcrossing mice from chimera.svg”. By Seans Potato Business – Own work  via C

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things

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