Introduction:
In genetics, the term "dominant" refers to a gene that is expressed and produces a specific trait or characteristic when present in an individual's genetic makeup. The concept of dominance is crucial in understanding how genetic traits are inherited and passed on from one generation to the next.

Explanation:
The correct answer to the given question is option 'C' - a gene is said to be dominant if it expresses its effect both in homozygous and heterozygous conditions. Here's an explanation of why this answer is correct:

Homozygous Dominance:
When a gene is said to be "dominant," it means that the presence of even a single copy of that gene is sufficient to express its effect. In the homozygous condition, where an individual carries two copies of the same gene (one from each parent), a dominant gene will always be expressed. For example, if a person carries two copies of the dominant gene for brown eyes (BB), they will have brown eyes.

Heterozygous Dominance:
In the heterozygous condition, where an individual carries two different alleles (gene variants) for a particular gene, a dominant gene will still be expressed. This means that if an individual carries one copy of the dominant gene and one copy of a recessive gene (Bb), the dominant gene will mask the expression of the recessive gene, and the dominant trait will be observed. For example, if a person carries one dominant gene for brown eyes and one recessive gene for blue eyes, they will still have brown eyes because the dominant gene is expressed.

Recessive Genes:
In contrast to dominant genes, recessive genes are only expressed when an individual carries two copies of the recessive allele (bb) in the homozygous condition. If an individual carries one copy of the recessive allele and one copy of the dominant allele (Bb), the dominant gene will be expressed, and the recessive gene will remain hidden. This is why individuals with heterozygous genotypes for a recessive gene are called carriers, as they carry the recessive allele but do not express the associated trait.

Conclusion:
In summary, a gene is said to be dominant if it expresses its effect both in homozygous and heterozygous conditions. This means that the presence of a dominant allele will always result in the expression of its associated trait, regardless of whether the individual is homozygous or heterozygous for that gene.