Which of the following best describes a recessive trait?

A recessive trait is characterized by being expressed in the phenotype only when two copies of the recessive allele are present, meaning that its expression occurs only in the absence of a dominant allele. When a dominant allele is present, it masks the effect of the recessive allele, preventing the recessive trait from being observed in the phenotype.

This definition highlights the crucial aspect of dominance and recessiveness within genetic inheritance. For instance, in a genetic scenario where a dominant allele for a trait (like brown eyes) is present alongside a recessive allele for a different trait (like blue eyes), only the dominant trait will manifest in the individual’s appearance, unless the individual has two copies of the recessive allele (homozygous recessive).

The other options describe different concepts in genetics. Some suggest that traits are expressed in all instances regardless of dominance, which is not accurate for recessive traits, while others imply that recessive traits might always cause harmful effects, which is not characteristic of all recessive alleles. Thus, the assertion that a recessive trait only appears when the dominant allele is absent is the most accurate representation of what constitutes recessiveness in genetic terms.