Why do all plants appear tall in the first generation when pure tall and pure dwarf pea plants are crossed?

Correct answer: Because the tall trait is dominant

Let the allele for tallness be T and the allele for dwarfness be t. A pure tall plant has TT and a pure dwarf plant has tt. Their cross produces F1 offspring with genotype Tt. Since T is dominant, each heterozygous plant expresses tallness even though the recessive t allele is present. Thus option A is correct; the dwarf allele is masked, not destroyed.