What do we call a trait that only appears when an individual has two alleles for it?

A trait that only appears when an individual possesses two alleles for it is referred to as a recessive trait. This means that in order for the physical expression of the trait to occur, an individual must inherit two copies of the allele—one from each parent. In contrast, dominant traits can manifest even when only one copy of the dominant allele is present.

Understanding this concept is crucial because it reflects how traits are transmitted from parents to offspring and helps explain genetic variations within populations. In cases where only one allele for a trait is dominant, the recessive trait remains hidden in the presence of the dominant allele unless an individual is homozygous for the recessive trait. Thus, recessive traits require both alleles to be identical and recessive in nature for the characteristic to be expressed in the phenotype.