The dominant allele will always be shown in the phenotype if it's present, because the other allele would be dominant or recessive. If both alleles are dominant, then the dominant one shows, if one of the alleles is recessive, then the dominant allele would show over the recessive one.
I think of recessive alleles as "weak" where, they just follow behind whichever other allele exists there. If the other one is dominant, then the recessive one is behind it, and no one can see it. If it other allele is recessive, a recessive allele will be in front.
