A gene that is always expressed if present is called a dominant gene.

Here's why:

* Dominant genes exert their effect even if only one copy of the gene is present in an individual's genotype.

* Recessive genes, on the other hand, only exert their effect if two copies of the gene are present.

Example:

* Brown eyes are a dominant trait. If you inherit one gene for brown eyes and one for blue eyes, you will have brown eyes because the brown eye gene is dominant.

Let me know if you have any other questions about genetics!