Question

Are dominant alleles always expressed in the phenotype?

Answer 1

Yes, but the penetrance of the allele can differ.

Explanation:

Alleles that exhibit complete dominance will always be expressed in the the cell's phenotype. However, sometimes dominance of an allele is incomplete. In that that case, if a cell has one dominant and one recessive allele (i.e. heterozygous), the cell can display intermediate phenotypes. It is called co-dominance when two different alleles contribute equally to the phenotype.

So the dominant allele is always expressed, but it can differ in penetrance and is therefore not always as clearly expressed in the cell's phenotype as you expect from a dominant allele.

The information on the following website might be of help to you:
Link to SciencePrimer