What describes an allele that is only expressed when two copies are present?

An allele that is only expressed when two copies are present is known as a recessive allele. This means that for the traits associated with this allele to be observed in an organism, it must inherit two identical copies of that allele—one from each parent. Recessive alleles do not manifest their effects if a dominant allele is present, as the dominant allele will overshadow the expression of the recessive one.

In contrast, a dominant allele can express its traits even when there is only one copy present. The term heterozygous refers to an organism that has two different alleles for a particular gene, and codominant describes a situation where both alleles in a heterozygous pair are fully expressed. Therefore, the defining feature of a recessive allele is its requirement for two copies to be phenotypically visible.