How can the concept of dominance be explained?

(A) To understand dominance,we must consider how genes function. $A$ gene contains the information required to express a particular trait.
In diploid organisms,each gene exists as a pair of alleles. These alleles may not always be identical; they can be heterozygous. $A$ variation in one allele often arises from a mutation that modifies the information it carries.
Consider a gene that codes for an enzyme. The two alleles of this gene are its two forms. The normal allele produces the enzyme required for the transformation of a substrate '$S$'.
The modified allele could be responsible for: $(i)$ a normal or less efficient enzyme,$(ii)$ a non-functional enzyme,or $(iii)$ the absence of the enzyme.
In the first case,the modified allele is equivalent to the unmodified allele,resulting in the same phenotype. Thus,the transformation of substrate '$S$' occurs normally.
However,if the modified allele produces a non-functional enzyme or no enzyme at all,the phenotype may be affected. The phenotype depends on the functioning of the unmodified allele.
The allele that produces the functional enzyme,which determines the original phenotype,is considered 'dominant',while the modified allele is considered 'recessive'.