What is the name of an allele that requires two copies to be expressed?

An allele that requires two copies to be expressed is referred to as a recessive allele. In genetic terms, for a trait governed by a recessive allele to manifest in an organism, both alleles at a corresponding gene locus must be the recessive variant. This means that the individual must inherit one recessive allele from each parent.

For example, consider a situation where a gene controls flower color in a plant. If purple is dominant and white is recessive, only plants that have two copies of the white allele (white/white) will exhibit white flowers. Plants with at least one copy of the purple allele (purple/purple or purple/white) will show the purple phenotype because the dominant allele masks the expression of the recessive one.

On the other hand, heterozygous refers to having two different alleles for a particular gene, dominant indicates an allele that can express its trait even when only one copy is present, and polymer is not a relevant term in this context of genetics. Together, these definitions help clarify why the correct answer is associated with the need for two copies to see the trait expressed.