In a typical test cross -

The correct answer is Option (2) → An organism showing a dominant phenotype is crossed with the recessive parent

A tall pea plant may be TT or Tt; its appearance does not show which. In the test cross the recessive parent (tt) gives only t gametes, so the progeny show the test plant’s genotype:

• TT × tt → all Tt, all tall: the test plant is homozygous.

• Tt × tt → 1 Tt : 1 tt, half tall and half dwarf: the test plant is heterozygous.

Option (1) An organism showing a recessive phenotype is crossed with the dominant parent — Incorrect. The roles are reversed. A recessive phenotype already shows its genotype (tt), so there is nothing to test.

Option (2) An organism showing a dominant phenotype is crossed with the recessive parent — Correct. This is the test cross. The recessive parent reveals the hidden allele of the organism being tested.

Option (3) An organism showing a dominant genotype is crossed with the dominant parent — Incorrect. Crossing with a dominant parent gives all dominant-looking progeny, so TT and Tt cannot be told apart.

Option (4) An organism showing a recessive genotype is crossed with the recessive parent — Incorrect. tt × tt gives only tt progeny. The genotype is known already, so this tests nothing.

Note: A similar question was also asked in CUET (UG) 2025 on 22-05-2025 (Slot 2) and on 14-05-2025 (Slot 1).

NCERT Extract:
Inheritance of One Gene – …