The typical test cross is performed between -

The correct answer is Option (1) → An organism showing a dominant phenotype and the recessive parent

A plant showing the dominant trait (tall) may be TT or Tt; the two look the same. A test cross finds out which, by crossing it with the recessive parent (tt), which can give only t gametes.

• TT × tt gives all Tt: all offspring tall, so the plant was TT.

• Tt × tt gives Tt and tt: tall and dwarf in 1 : 1, so the plant was Tt.

Option (1) An organism showing a dominant phenotype and the recessive parent — Correct. Only the recessive parent lets every allele of the test plant show in the offspring.

Option (2) An organism showing a dominant phenotype and the dominant parent — Incorrect. A TT parent gives T to every offspring, so all would look dominant and the hidden allele could not be seen.

Option (3) An organism showing a dominant genotype and the dominant parent — Incorrect. If the genotype is already known, there is nothing left to test; and crossing with the dominant parent again hides the recessive allele.

Option (4) An organism showing a recessive phenotype and the dominant parent — Incorrect. A plant with the recessive phenotype must be tt, so its genotype needs no test.

Note: A similar question was also asked in CUET (UG) 2025 on 30-05-2025 (Slot 1).

NCERT Extract:
Inheritance of One Gene – …