The key difference between test cross and backcross is that the test cross is the cross that occurs between a dominant phenotype and a recessive phenotype while the backcross is the cross that occurs between generation F1 hybrid and one of the two parents.
Understanding the difference between test cross and backcross is important in genetics as they are two different types of crosses that are extremely helpful to identify the genotype of an animal or a plant. The major objective of performing the test cross and backcross is to discover the heterozygosity or homozygosity of individuals by identifying the types of gametes that produce the dominant genotypes.
Consider the following example to understand both crosses and the difference between test cross and backcross. Here, ‘T’ denotes the dominant trait of the tall pea plant, and ‘t’ denotes the recessive trait of the same phenotype. A tall pea plant hybrid can either exist as homozygous (TT) or heterozygous (Tt) and the dwarf plant hybrid is always homozygous recessive (tt).
CONTENTS
1. Overview and Key Difference
2. What is Test Cross
3. What is Backcross
4. Similarities Between Test Cross and Backcross
5. Side by Side Comparison – Test Cross vs Backcross in Tabular Form
6. Summary
What is Test Cross?
In test cross, the F1 hybrid is crossed back with the recessive parent. In other words, test cross is the cross between a dominant phenotype (TT or Tt) and a homozygous recessive (tt). Mendel was the first person to perform the test cross to identify whether an individual is heterozygous or homozygous for the dominant character. Other than discovering heterozygosity, test cross is also useful to check the purity of the gametes produced by parents.
If a homozygous dominant F1 hybrid (TT) crosses with the recessive parent, it will always result in 100% heterozygous tall hybrids. The below figure explains this.
If a heterozygous dominant F1 hybrid (Tt) crosses with the recessive parent, only 50% will be tall, and the rest 50% will be dwarf. The below image explains this.
What is Backcross?
In backcross, the F1 hybrid is crossed back with any of the parent, either dominant or recessive. Backcrosses increase the useful traits in a population. For example, certain crop plant hybrids are backcrossed with wild species to recover their useful traits such as disease resistance, high yield, etc.
Figure 03: Backcross
However, this process may dilute the other useful traits of hybrid. In order to overcome this disadvantage, hybrids are backcrossed repeatedly with its parent plants over a few generations to receive their good traits back into the new hybrids.
What are the Similarities Between Test Cross and Backcross?
- Test cross and backcross are useful in plant and animal breeding.
- They explain the phenotypes and genotypes of an organism and how they pass to next generation.
- All test crosses are backcrosses.
- They determine the genotype of an individual as well as help to recover important characteristics.
- In both, the crossing is between unknown genotypes.
What is the Difference Between Test Cross and Backcross?
Test cross and backcross are two types of popular crosses in plant breeding. Test cross happens between a dominant phenotype with the recessive phenotype to determine the genotype of the dominant phenotype. Backcross helps to recover important characters of the parent population within hybrid populations.
Summary – Test Cross vs Backcross
All test crosses are a type of backcross, but all back crosses are not test cross. During the backcross, the crossing back of the F1 hybrid is with any of the parents, either homozygous or heterozygous. However, during the test cross, the crossing back of the F1 hybrid is always with the recessive parent. A test cross is important to determine the genotype (TT or Tt) of the dominant phenotype while backcross is useful in recovering important characteristics of the parent. This is the difference between a test cross and backcross.
Reference:
1.“Backcrossing.” Egyptian Journal of Medical Human Genetics, Elsevier. Available here
2.Nature News, Nature Publishing Group. Available here
Image Courtesy:
1.”Inbreeding- Backcross”By Donach – Own work, (CC BY-SA 4.0) via Commons Wikimedia
So can a test cross be between a F2 and a recessive parent Or is it restricted to F1 only?
Test cross is always done for the determination of zygosity (homozygous or heterozygous) of a dominant phenotype. Hence, a particular dominant phenotype is crossed with its recessive phenotype. The dominant phenotype can be obtained either from F1 or F2. However, the phenotype subjected to a test cross should be dominant.