In pea, a pure tall plant (TT) is crossed with a short plant (tt). What will be the ratio of pure tall plants to short plants in the F₂ generation ?

The correct answer is 1: 1.

Key Points

• '​Pure' plants refer to the homozygous condition in plants and 'hybrid' refers to the heterozygous plants.
• Homozygous - It is the condition in which the pair of alleles determining a phenotype is identical, either both dominant (TT) or both recessive (tt).
• Heterozygous - It is the condition where 2 different types of alleles are present, one dominant and one recessive (Tt).
• When a pure tall plant (TT) is crossed with a pure short plant (tt), the F1 generation produces heterozygous tall (Tt) plants.
• Upon selfing the F1 generation, that is, crossing two heterozygous tall plants, the following numbers of the F2 generation are obtained phenotypically:
  • Three tall plants and one short plant
• Whereas, genotypically, the following ratio is obtained:
  • One pure tall plant, two heterozygous tall plants, and one pure short plant.
• Therefore, considering the genotype, the ratio of pure tall plants to pure short plants in the F2 generation is 1:1. Hence, Option 1 is correct.

Mistake Points

• Please note here that the question asks about the ratio of "pure" tall plants to short plants in F2 generation.
• The usual ratio of 3:1 is for "all" tall plants to short plants.
• Pure Tall plants : Hybrid Tall plants : Short/Dwarf plants = TT : Tt : tt = 1:2:1
• Therefore, Pure tall : Pure short = 1:1