Test Cross

A test cross is the mating of an incompletely known genotype to a genotype which is homozygous, recessive at all the loci under consideration. The phenotypes of the offspring produced by a test cross reveal the number of different gametes formed by the parental genotype under test. When all of the gametes of an individual are known, the genotype of that individual is also known. A monohybrid test cross gives a 1: 1 phenotypic ratio (Fig. 10' 5 J, indicating that one pair of factors is segregating. A dihybrid test cross gives a 1 : 1 : 1 : 1 ratio, indicating that two pairs of factors are segregating and assorting independently. A trihybrid test cross, however gives a 1: 1: 1: 1: 1: 1: 1 : 1, ratio, while three pairs of factors are segregating and assorting independently.


Example:
1. Monohybrid Test Cross
Test crossing of Bb x bb produces the 1: 1 genotypic and phenotypic ratio as has been illustrated in following diagram.

2. Dihybrid Test Cross -

The test cross of a heterozygous yellow round seeded pea plant with a double recessive parent (green wrinkled. yy rr), yields the test cross genotypic and phenotypic ratio of 1: 1: 1:1 as follows:

3. Trihybrid Test Cross





A trihybrid test cross involves crossing of a trihybrid (Yy Rr Tt) with its triple recessive (yyrrtt) parent and it yields the test cross genotypic and phenotypic ratio-of 1:1:1:1:1:1:1:1 as follows:

F1: 1/8 Yy Rr Tt (Yellow, Round, Tall): 1/8 Yy Rr tt (Yellow, Round, Dwarf): 1/8 Yy rr Tt (Yellow, wrinkled, Tall): 1/8 Yyrrtt (Yellow, Wrinkled, Dwarf):1/8 yyRrTt (Green, Round, Tall): 1/8 yy Rr tt (Green, Round, Dwarf): 1/8 yy rr Tt (Green, wrinkled, Tall): 1/8 yy rr tt (Green, wrinkled, Dwarf).