What is the genotypic ratio of the monohybrid cross conducted by Mendel?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1:2:1
A monohybrid cross follows one character, such as height in the pea plant. Mendel crossed a pure tall plant (TT) with a pure dwarf plant (tt). All F1 plants were tall, with the genotype Tt. Each F1 plant forms two kinds of gametes, T and t, in equal numbers. When F1 plants are self-pollinated, the F2 generation is:
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
So the genotypes are 1 TT : 2 Tt : 1 tt, a genotypic ratio of 1:2:1. Because TT and Tt plants both look tall, the phenotypic ratio is 3 tall : 1 dwarf (3:1). The ratio 9:3:3:1 belongs to a dihybrid cross, which follows two characters.
NCERT Extract:
Inheritance of One Gene – “… From the Punnett square it is easily seen that 1/4th of the random fertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt. Though the F1 have a genotype of Tt, but the phenotypic character seen is ‘tall’. … 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.”