In learning genetics, many students misunderstand and misinterpret what “dominance” means. Understanding is easier if students realize that dominance is not a mechanism, but rather a consequence of underlying cellular processes. For example, metabolic pathways are often little affected by changes in enzyme concentration. This means that enzyme-producing alleles usually show complete dominance. For genes producing nonenzymatic proteins such as collagen or hemoglobin, the amount of product matters, and dominance relationships are more complicated. Furthermore, with hemoglobin, dominance can change depending on what aspect of the phenotype is being studied and on the environmental conditions. X-linked genes are a special case, whether enzymatic or not. Because of X-chromosome inactivation, only one X-linked allele can be active in a cell, which means that the concept of dominance cannot be applied at the cellular level. Instead, a type of dominance is demonstrated at the individual level; but even so, dominant traits may fail to be expressed, and recessive traits can be expressed. Teaching not only what is happening but why it's happening will give students a deeper understanding, not only of dominance relationships, but of the underlying cellular processes as well.
University of California Press