This was a question, I have been asked yesterday.
Although dominance is the property an allele, dominance is not”regulated” on the genomic level but a function of the resulting protein. According to the largely citied Wilkie paper there are numerous mechanisms
- reduced gene dosage expression or protein activity
- increased gene dosage
- ectopic or temporally altered mRNA expression
- increased or constitutive protein activity
- dominant negative effects
- altered structural proteins
- toxic protein alterations
- new protein functions
In lay terms also explained at biology.stackexchange
… the dominant allele encodes a protein that can perform its function. For example, the dominant allele for the CFTR gene encodes a channel that can let chloride into and out of the cells. The recessive allele, on the other hand encodes a protein that cannot do its job correctly (this also called a loss-of-function mutation). So if you inherit a functional copy from one parent and a non-functional copy from the other parent, you will still have one copy of the protein that can do its job. Only if you get a nonfunctional copy from both parents will you have a recessive condition called cystic fibrosis.