The gene that is masked when paired with a dominant gene is called the recessive gene.

Here's a simple explanation:

* Dominant genes express their trait even when paired with a recessive gene. They're like the "stronger" gene.

* Recessive genes only express their trait when paired with another recessive gene. They're like the "weaker" gene that gets masked.

Example:

Let's say "B" represents the dominant gene for brown eyes and "b" represents the recessive gene for blue eyes.

* BB: Brown eyes (dominant gene paired with dominant gene)

* Bb: Brown eyes (dominant gene paired with recessive gene)

* bb: Blue eyes (recessive gene paired with recessive gene)

In the second case (Bb), the blue eye gene (b) is masked by the brown eye gene (B), resulting in brown eyes.