Here's why:

* Recessive alleles only express their trait if two copies of the allele are present. This means that the individual must inherit one copy of the recessive allele from each parent.

* Dominant alleles, on the other hand, will express their trait even if only one copy is present.

Example:

Let's say brown eyes (B) are dominant to blue eyes (b).

* A person with the genotype BB or Bb will have brown eyes.

* A person with the genotype bb will have blue eyes.

In this example, blue eyes are the recessive trait because you need two copies of the blue eye allele (b) to express the blue eye color.