What defines a trait that is only expressed if two recessive alleles are present?

A trait that is only expressed when two recessive alleles are present is defined as a recessive trait. This means that for the trait to be observable in the phenotype, an individual must inherit two copies of the recessive allele, one from each parent.

In contrast, a dominant trait can be expressed even if only one copy of the dominant allele is present, overshadowing the recessive trait. Sex-linked traits are associated with genes located on sex chromosomes and their expression can depend on the sex of the individual, but they do not solely relate to whether an allele is dominant or recessive. Homozygous refers to having two identical alleles at a locus, but it doesn't specifically indicate whether those alleles are dominant or recessive. Therefore, identifying a trait as recessive highlights the necessity of having two of its specific alleles for expression in an organism’s phenotype.