Question

Dihybrid test cross ratio is
A. 9:9:3:1
B. 1:1:1:1
C. 3:1
D. 1:1

Answer 1

In a test cross, an individual whose zygosity is to be known is crossed with a homozygous recessive individual. The phenotypic ratio of the cross helps in determining the zygosity of the crossed individual easily.

Complete answer:
For this, let us first discuss what a test cross is. A test cross is basically done to know about the zygosity of the individual whether it is a homozygous individual or a heterozygous individual. For this, the individual is then crossed with the homozygous recessive individual of the same species. The phenotypic ratio of the cross is then analyzed, and it is further contradicted whether the individual is homozygous or recessive.
The concept of test cross was given by Gregor Mendel when he gave the laws of inheritance by experimenting upon the pea plant. In a dihybrid cross, two different characters of the individual are taken into consideration knowing about which one is recessive and which one is dominant. But in the case of monohybrid cross, only one character is taken into consideration. The ratio of a monohybrid test cross is 1:1 if the individual is heterozygous and if the individual is homozygous then the ratio is 1.
For the dihybrid cross if the individual is homozygous then the phenotypic ratio will be the same as that of the monohybrid cross that is 1 but if the individual is heterozygous then the ratio will be 1:1:1:1.

Therefore, the correct answer is the option B.

Note:
The reason for the phenotypic ratio of the homozygous individual with the recessive one is Because the phenotype will remain the same in all cases as the dominant gene will express itself, the difference will occur only in the case of heterozygous as there are different alleles.