Test Cross vs Backcross
Understanding the difference between test cross and backcross is important as test cross and backcross are two types of crosses used in genetics that are extremely helpful to identify the genotype of an animal or a plant. The major objective of performing test cross and backcross is to discover the heterozygosity or homozygosity of individuals by identifying the types of gametes that produce the dominant genotypes. The following example will be considered to explain test cross and back cross and the difference between the two types of crosses, in this article.
The dominant trait for tall pea plant is denoted as T and the recessive trait for the same phenotype is denoted as t. A tall pea plant hybrid can either exist as homozygous (TT) or heterozygous (Tt) and dwarf plant hybrid is always homozygous recessive (tt).
What is Test Cross?
In test cross, 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 dominant character. Other than discovering heterozygosity, test cross is also useful to check the purity of the gametes produced by parents.
If homozygous dominant F1 hybrid (TT) is crossed with the recessive parent, it will always result receive 100% heterozygous tall hybrids.
If heterozygous dominant F1 hybrid (Tt) is test crossed with the recessive parent, only 50% will be tall and the rest 50% will be dwarf.
What is Backcross?
In backcross, F1 hybrid is crossed back with any of the parent, either dominant or recessive. Back crosses are used to increase useful traits in a population. For example, certain crop plant hybrids are backcrossed with wild species to obtain its useful traits such as disease resistance. 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 few generations to receive their good traits back into the new hybrids.
What is the difference between Test Cross and Backcross?
• All test crosses are considered as backcrosses, but all backcrosses are not the test crosses.
• During backcross, F1 hybrid is crossed back with any of the parents, eit her homozygous or heterozygous. However, during test cross, F 1 hybrid is always crossed back with the recessive parent.
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.