The allele whose trait always shows up in the organism is called a dominant allele.

Here's why:

* Dominant alleles mask the effects of recessive alleles. This means that even if an organism has one dominant allele and one recessive allele, the dominant trait will be expressed.

* Recessive alleles only show up in the organism's phenotype (observable traits) if two copies of the recessive allele are present.

For example, brown eyes are dominant to blue eyes. Someone with one brown eye allele and one blue eye allele will still have brown eyes because the brown allele is dominant.