An allele is described as 'dominant' if it expresses itself when it occurs in

The correct option is C Both heterozygous and homozygous conditions
Hemizygous condition refers to the presence of single copy on a gene rather than two as are present for mendelian traits. Since the dominance-recessive relationship is applied when a trait is exhibited by two copies of same or different alleles of a gene, both recessive and dominant traits are expressed in a homozygous condition which is characterized by the presence of two copies of same alleles for a gene in an individual (TT or tt). The presence of contrasting alleles of a gene in an individual is referred to as heterozygous condition. For example, a plant with genotype “Tt” is heterozygous for the gene of height. Since expression of a recessive allele is masked by the presence of the dominant allele in heterozygous genotype, the only dominant trait is expressed by them irrespective of the presence of both dominant and recessive alleles. Thus, the correct answer is option C.