Assertion: In a monohybrid cross, F1 generation indicate dominant characters.
Reason: Dominance occurs only in heterozygous state.

Assertion (A) is true but Reason (R) is not correct.

The assertion that "In a monohybrid cross, F1 generation indicate dominant characters" is generally correct.

During a monohybrid cross involving two pure-breeding (homozygous) parents that differ in only one trait (e.g., one is homozygous dominant and the other is homozygous recessive), the F1 (first filial) generation typically displays the dominant trait. This is because the dominant allele will mask the expression of the recessive allele, resulting in the manifestation of the dominant phenotype in the offspring.

However, the reasoning provided, which states that "Dominance occurs only in the heterozygous state," is not accurate.

Dominance and recessiveness refer to the relationship between alleles at a particular gene locus. Dominance occurs when one allele (dominant allele) in a pair exerts its phenotypic effect over the other allele (recessive allele) in a heterozygous individual. This means that the dominant allele's trait is expressed in the phenotype when present in either the heterozygous or homozygous dominant state.

In heterozygous individuals, where both dominant and recessive alleles are present, the dominant allele masks the expression of the recessive allele, leading to the dominant trait being observed. However, in homozygous dominant individuals, where both alleles are of the dominant type, dominance is also exhibited as the dominant trait is expressed.

Therefore, while the assertion regarding the F1 generation in a monohybrid cross indicating dominant characters is generally true, the reasoning that dominance occurs only in the heterozygous state is not accurate. Dominance can be expressed in both heterozygous and homozygous dominant states.