A phenotypic trait that results from a single dominant allele is called a dominant trait.

Here's why:

* Dominant Alleles: These alleles exert their effect even if only one copy is present in an individual's genotype.

* Recessive Alleles: These alleles only exert their effect when two copies are present (one from each parent).

Example:

Let's say the gene for eye color has two alleles: brown (B) and blue (b). Brown is dominant over blue. This means:

* BB: Brown eyes

* Bb: Brown eyes (because the dominant B allele masks the b allele)

* bb: Blue eyes (because there are two copies of the recessive b allele)

So, in this case, having even one copy of the brown allele (B) will result in brown eyes, making it a dominant trait.