The terms "dominant" and "recessive" are used to describe the phenotypic expression of certain traits or characteristics an individual possesses and not the genes themselves. In humans, each gene has two copies, or alleles. A person with a dominant allele for a specific trait always expresses that trait, even if the other copy of the allele he possesses is recessive. Dominant alleles repress, or mask, recessive alleles.
A single copy of a dominant allele is enough to produce a dominant phenotype, or characteristic. A recessive phenotype is only expressed (or visible) if a person possesses two recessive copies of the allele, obtaining one from each parent. For example, a person only needs to have one copy of the dimples allele to possess this characteristic, because having dimples is a dominant trait, whereas not having dimples is recessive. Using “D” to represent the dominant allele for dimples and “d” for the recessive allele, a person would have dimples if that person possesses a “DD” or a “Dd” allele pair. However, a person who does not have dimples must possess the “dd” allele pair. A person possessing one dominant allele and one recessive allele for a gene is said to be heterozygous for that trait because the alleles are different. However, if a person possesses two alleles that are the same, such as two dominant alleles or two recessive alleles, then that person is homozygous for that trait.