1. Dominant: This form of the gene expresses its trait even if only one copy is present. It masks the expression of the recessive allele.

2. Recessive: This form of the gene only expresses its trait if two copies are present. It is masked by the dominant allele.

For example, the gene for eye color has two alleles: brown (dominant) and blue (recessive). A person with one brown allele and one blue allele will have brown eyes because the brown allele is dominant. However, a person with two blue alleles will have blue eyes because the recessive allele is expressed when there is no dominant allele present.