If Mendel had crossed a true breeding dominant plant with a true breeding recessive plant, in which of the three generations is the recessive trait visible?

The recessive parent in the P generation shows the recessive trait. Crossing true-breeding dominant and recessive plants produces F₁ offspring that all show the dominant trait, because they carry one dominant allele. When the F₁ plants are crossed, some F₂ offspring inherit two recessive alleles and show the recessive trait. So the trait is visible in the P and F₂ generations: choice C.

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