If allele is dominant it is expressed in both condition
but if allele is recessive then it express only in homozygous condition..

for example Tt,TT both r dominant allele n represent tall plants.

tt is recessive n represent dwarf plant..

Introduction:
An allele is a variant form of a gene that determines a specific trait. In genetics, alleles can be either dominant or recessive. The dominance of an allele refers to its ability to be expressed in the phenotype of an organism. In this question, we are asked to determine the condition under which an allele is dominant.

Explanation:
The correct answer is option A, which states that an allele is dominant if it is expressed in both homozygous and heterozygous states. This means that regardless of whether an individual carries two copies of the dominant allele (homozygous) or only one copy (heterozygous), the dominant allele will be expressed.

Here is a detailed explanation of why option A is the correct answer:

Homozygous State:
- In genetics, homozygous refers to an individual having two identical alleles for a particular gene.
- If the dominant allele is present in the homozygous state (AA), it will be expressed, and the individual will exhibit the trait determined by that allele.
- For example, if we consider a gene for eye color, where the dominant allele (A) determines brown eye color and the recessive allele (a) determines blue eye color, an individual with the genotype AA will have brown eyes.

Heterozygous State:
- Heterozygous refers to an individual having two different alleles for a particular gene.
- If the dominant allele is present in the heterozygous state (Aa), it will still be expressed, and the individual will exhibit the trait determined by that allele.
- In the eye color example, an individual with the genotype Aa will also have brown eyes because the dominant allele (A) is expressed.

Recessive Allele:
- In contrast, recessive alleles are only expressed in the homozygous state.
- In the eye color example, the recessive allele (a) for blue eye color will only be expressed if an individual carries two copies of it (aa).
- Therefore, the recessive allele is not dominant because it requires the homozygous state for expression.

Conclusion:
In summary, an allele is considered dominant if it is expressed in both the homozygous and heterozygous states. This means that individuals carrying either two copies or only one copy of the dominant allele will exhibit the trait determined by that allele. The recessive allele, on the other hand, is only expressed in the homozygous state.