When one or both copies of a gene are the ______ allele, then this allele's form of the trait will be exhibited.

In genetics, when one or both copies of a gene are the dominant allele, then this allele's form of the trait will be exhibited.

Dominant alleles are characteristic of having the ability to mask the effect of a recessive allele. In simple terms, if an individual has at least one dominant allele for a trait, that trait will be expressed phenotypically, meaning it will be visible in the organism's appearance. For example, if the allele for purple flowers (P) is dominant over the allele for white flowers (p), a plant with either the genotype PP or Pp will exhibit purple flowers, whereas only the genotype pp will exhibit the recessive trait, white flowers.

This concept is foundational in the field of genetics and is a key factor in understanding inheritance patterns, the principles of Mendelian genetics, and the way traits are passed down through generations.