When an allele of a gene masks the presence of another allele, the dominant form of the allele is seen in the phenotype.

Here's a breakdown:

* Allele: A variation of a gene.

* Dominant Allele: An allele that exerts its effect even when paired with a different allele.

* Recessive Allele: An allele whose effect is masked by the dominant allele.

* Phenotype: The observable characteristics of an organism.

Example:

Let's say a gene controls flower color. There are two alleles:

* B: Dominant allele for blue flowers

* b: Recessive allele for white flowers

* BB: Blue flowers (dominant allele expressed)

* Bb: Blue flowers (dominant allele masks the recessive allele)

* bb: White flowers (recessive allele expressed only when paired with another recessive allele)

In this case, even if an organism has one copy of the dominant allele (B) and one copy of the recessive allele (b), it will still express the dominant phenotype (blue flowers).