Which trait requires two copies to be expressed?

A trait that requires two copies to be expressed is termed recessive. For a recessive trait to manifest in an organism, both alleles for that trait must be present. This means that the organism must inherit one recessive allele from each parent.

In contrast, dominant traits require only one copy of the allele to be expressed, meaning that having just one dominant allele can overshadow the recessive one. Homozygous refers to the genetic condition of having two identical alleles for a trait, which can describe either dominant or recessive traits, while heterozygous refers to having two different alleles for a trait. Therefore, the specific characteristic of requiring two copies to be expressed aligns clearly with the definition of a recessive trait.