Which one of the following ratios belongs to the monohybrid test cross?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1:1
A test cross is a cross of an organism that shows the dominant trait (and whose genotype is not known) with the homozygous recessive parent. The offspring show what gametes the test organism makes. If the test organism is heterozygous (Tt), half of the offspring show the dominant trait and half the recessive trait. If it is homozygous (TT), all offspring show the dominant trait.
Parents: tall Tt × dwarf tt. Gametes: T and t from the tall plant, only t from the dwarf plant.
| T | t | |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |
Phenotypic ratio: 1 tall : 1 dwarf. Genotypic ratio: 1 Tt : 1 tt. So both ratios are 1:1.
(1) 9:3:3:1 — Incorrect. This is the phenotypic ratio in the F2 of a dihybrid cross.
(2) 1:1 — Correct. This is the ratio of a monohybrid test cross with a heterozygous parent.
(3) 3:1 — Incorrect. This is the phenotypic ratio in the F2 of a monohybrid cross.
(4) 1:2:1 — Incorrect. This is the genotypic ratio in the F2 of a monohybrid cross.
NCERT Extract:
Inheritance of One Gene – “… In a typical test cross an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analysed to predict the genotype of the test organism.”