Comprehension:
Mendelian genetics
Read the following passage carefully and answer the given questions.
In Mendel's monohybrid cross between pure tall (TT) and pure dwarf (tt) pea plants, the F1 generation consisted of all tall plants, including dominance of T over t. The tall and dwarf traits were identical to their parental type and did not show any blending. Selfing of F1 plants produced F2 generation in the ratio of 3 Tall: 1 dwarf, whereas genotypic ratio was 1TT : 2Tt : 1tt. Mendel concludes that alleles segregate during gamete formation and one allele from each parent is transmitted to offspring. Also, based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. Now we call them as genes as unit of inheritance. The production of gametes by the parents, the formation of the zygotes, the F1 and F2 plants can be understood from a diagram called Punnett Square.
Identify the genotypic ratio in F2 generation when offspring of first filial generation of pure tall and pure dwarf were self crossed.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 1:2:1
The F1 plants are all Tt. Each makes two kinds of gametes in equal numbers, T and t. When F1 plants are selfed, these gametes combine at random:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
The square gives 1 TT : 2 Tt : 1 tt. So the genotypic ratio is 1:2:1. The phenotypic ratio is 3 tall : 1 dwarf, because TT and Tt are both tall.
(1) 1:2:1 — Correct. One quarter TT, one half Tt and one quarter tt.
(2) 1:1:1 — Incorrect. The cross gives four combinations, but these three parts make only three.
(3) 1:1:2 — Incorrect. The double share belongs to the heterozygote Tt, which forms in two ways, and not to tt.
(4) 2:1:1 — Incorrect. TT forms in only one way, so it cannot be the double share.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Inheritance of One Gene – “… Externally it is not possible to distinguish between the plants with the genotypes TT and Tt. Hence, within the genopytic pair Tt only one character ‘T ’ tall is expressed. Hence the character T or ‘tall’ is said to dominate over the other allele t or ‘dwarf’ character. It is thus due to this dominance of one character over the other that all the F1 are tall (though the genotype is Tt) and in the F2 3/4th of the plants are tall (though genotypically 1/2 are Tt and only 1/4th are 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.”