The form of a gene that always has its effect when present is called a dominant allele.

Here's why:

* Alleles are different versions of the same gene.

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

* Recessive alleles only exert their effect if two copies are present.

For example, in the case of eye color, the brown eye allele (B) is dominant over the blue eye allele (b). This means:

* BB: Brown eyes (dominant allele present twice)

* Bb: Brown eyes (dominant allele present once)

* bb: Blue eyes (recessive allele present twice)

Let me know if you'd like more examples or have further questions!