When a tall pea plant is crossed with a dwarf pea plant, what will be the genotypic ratio of the F2 generation after selfing of the F1 plants?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1:2:1
A genotypic ratio counts the gene combinations (TT, Tt, tt), not the look of the plants. Tall (T) is dominant over dwarf (t).
- Parents: tall TT × dwarf tt. Gametes: T and t.
- F1: all Tt (tall).
- Selfing of F1: Tt × Tt. Each parent gives gametes T and t in equal numbers.
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
- F2: 1 TT : 2 Tt : 1 tt, so the genotypic ratio is 1:2:1.
Option (1) 3:1 — Incorrect. 3:1 is the phenotypic ratio of the same F2: 3 tall (TT and Tt look alike) to 1 dwarf. The question asks for the genotypic ratio.
Option (2) 1:2:1 — Correct. TT, Tt and tt appear in the proportion 1/4 : 1/2 : 1/4. This is the genotypic ratio of a monohybrid cross.
Option (3) 9:3:3:1 — Incorrect. 9:3:3:1 is the phenotypic ratio of a dihybrid cross, where two genes (two characters) are followed together. Here only one gene, for height, is studied.
NCERT Extract:
Inheritance of One Gene – “… Externally it is not possible to distinguish between the plants with the genotypes TT and Tt. … This leads to a phenotypic ratio of 3/4th tall : (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1. The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is mathematically condensable to the form of the binomial expression (ax +by)2, that has the gametes bearing genes T or t in equal frequency of ½.”
Figure 4.4 A Punnett square used to understand a typical monohybrid cross conducted by Mendel between true-breeding tall plants and true-breeding dwarf plants