Because in a cross between two tall pea plants some of the offsprings produced were dwarf, it indicates that parent pea plants were heterozygous for tallness (Tt) i.e., they contain a recessive gene (t) for dwarfness and the dwarf plant (tt) recessive gene alleles for dwarfness from each of the parent plants.
25% plants were dwarf phenotypic ratio Tall : Dwarf :: 3 : 1 Genotypic ratio 1 :2 :1.
Punnett square:
Punnett square is a graphic representation of the probabilities of all the possible genotypes and phenotypes of offsprings in a cross.
