An allele that expresses itself in a hybrid is called a dominant allele.

Here's a breakdown:

* Allele: A variation of a gene. For example, the gene for eye color can have alleles for brown eyes, blue eyes, green eyes, etc.

* Hybrid: An organism that has two different alleles for a particular trait.

* Dominant Allele: An allele that masks the effect of the other allele in a hybrid.

Example:

Let's say brown eyes (B) are dominant to blue eyes (b). If a person inherits one B allele and one b allele (Bb), they will have brown eyes because the B allele is dominant.

In contrast, a recessive allele is only expressed if an individual has two copies of it (e.g., bb).