People are diploid.
That is, they possess two sets of chromosomes. The reason I must know about your parents is that when a particular gene has a dominant allele, a person with a genotype with one allele of the dominant gene will have the same phenotype as a person with two copies of the dominant allele. If the homozygous person has the dominant allele, they will have the same phenotype as the heterozygous person. So if you have a dominant phenotype, you could be either heterozygous or homozygous for your genotype. To determine which you are, I will have to consider the possble alleles you might have inherited from your parents.