Assertion (A) : In a monohybrid cross between tall plants and dwarf plants, the F2 generation showed tall and dwarf plants in the ratio of 3 : 1. Reason (R) : There is no blending of traits/characters in the F2generation.

The correct answer is Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

Analysis of Assertion (A):
In his experiments on pea plants (Pisum sativum), Gregor Mendel performed a monohybrid cross studying a single character: plant height. He crossed a pure tall plant with genotype
$TT$
and a pure dwarf plant with genotype
$tt$
In the first filial ($F_1$) generation, all the offspring were tall with the genotype
$Tt$
When these plants were self-pollinated, the second filial ($F_2$) generation yielded both tall and dwarf plants in a phenotypic ratio of 3 : 1 (75% tall and 25% dwarf). Thus, Assertion (A) is true.

Analysis of Reason (R):
The reason why the dwarf trait reappears unchanged in the F2 generation is because alleles do not blend or mix when they are present together in the heterozygous state of the F1 generation. During gamete formation, these alleles segregate or separate from each other completely cleanly. Each gamete receives only one of the two alleles. This ensures that the alleles for tallness
$T$
and dwarfness
$t$
maintain their individual identity. When they recombine in the F2 generation to produce homozygous
$tt$
plants, the parental dwarf phenotype is recovered in its original, unblended form. Therefore, Reason (R) is true.

How Reason (R) explains Assertion (A):
If blending of traits had occurred, the alleles would have merged in the heterozygous F1 generation, resulting in plants of intermediate height, and the distinct parental dwarf trait would never have reappeared in the F2 generation. Because there is no blending, the alleles segregate independently during gametogenesis, resulting in gametes carrying either allele in equal proportions. The random fusion of these gametes produces genotypes in the ratio of:
$1:2:1$
Since the allele for tallness is completely dominant, this genotypic combination results in the classic 3 : 1 phenotypic ratio of tall to dwarf plants. Hence, the absence of blending of traits is the biological reason that directly explains the segregation and the resulting phenotypic ratio.