What kind of trait is expressed only when two recessive alleles are present?

The trait that is expressed only when two recessive alleles are present is classified as a recessive trait. This occurs because recessive alleles do not manifest their associated characteristics in the presence of a dominant allele. Typically, for a recessive trait to be observable in an organism's phenotype, both alleles for that trait must be of the recessive type, as the presence of even a single dominant allele will overshadow the expression of the recessive one.

In this context, a dominant trait would manifest with at least one dominant allele present and does not require two recessive alleles. The term phenotypic relates to the observation of traits or characteristics that arise from the expression of alleles, and generational pertains to the lineage or progression of traits through generations, which does not specifically define how traits are expressed based on their allele combinations. Thus, the concept of recessiveness distinctly points to the necessity of two recessive alleles for expression, making it the correct choice.