Question

An organism with a homozygous recessive genotype is made to undergo a test cross which results in 50% of the offspring exhibiting the recessive trait. The test cross parent is:

I. Homozygous recessive

II. Homozygous dominant

III. Heterozygous

• A. I only
• B. II only
• C. III only
• D. I or III
• E. II or III

Answer 1

Answer: (C)

III only

A test cross is performed to determine the genotype of a dominant parent if it is a heterozygous or homozygous dominant. For the purpose, a dominant parent is crossed with the homozygous recessive parent.

Since the offspring from test cross has 50% recessive phenotypes, the test cross parent is heterozygous and fusion of gamete having the recessive allele with that of recessive parent gives 50% recessive progeny.

If the test parent was homozygous dominant, all progeny would exhibit only dominant phenotype.

Therefore, the correct answer is option C.