If a tall pea plant is crossed with a dwarf pea plant and all first-generation plants are tall, which trait is dominant?

Correct answer: Tallness

In a Mendelian cross, a dominant trait is expressed in the heterozygous first-generation offspring, whereas a recessive trait can remain hidden. Let T represent tallness and t represent dwarfness. A pure cross TT × tt produces F1 offspring with genotype Tt; because T is dominant, all F1 plants are tall. Therefore option A, tallness, is correct. Dwarfness is the contrasting recessive trait, and seed shape or colour is unrelated to this cross.