Question

A dihybrid for a qualitative trait is crossed with a homozygous recessive individual of its type, the phenotypic ratio is:

• A. 1 : 2 : 1
• B. 3 : 1
• C. 1 : 1: 1 : 1
• D. 9 : 3 : 3 : 1

Answer 1

The correct option is C 1 : 1: 1 : 1

• A cross between an organism with an unknown genotype and a recessive parent is known as a test cross. So, the given condition can be considered as a test cross.
• A dihybrid testcross gives a 1:1:1:1 ratio indicating that the two pairs of factors are segregating and assorting independently.
• Let us consider TtRr as the dihybrid for a qualitative trait and tr for the homozygous recessive individual.
• TtRr produces four types of gametes: TR, Tr, tR, tr.
• ttrr produces only one type of gamete: tr.
• A cross between them produces the following genotypes and phenotypes:

TtRr - tall, round, Ttrr - tall, wrinkle, ttRr - dwarf, round, ttrr - dwarf, wrinkle

• Hence, the phenotypic ratio observed is 1:1:1:1.

Thus, the correct answer is option C.