Module section description: It is important to remember that traits can be influenced by alterations in coding sequences and by alterations in gene regulation. In addition, some traits are influenced by a single gene and others are polygenic.
Another complicating factor is that the penetrance (the likelihood that a particular allele will actually produce a particular trait) of genetic mutations is often far less than 100%. Some environmental factors affect gene regulation and other environmental factors directly affect the development of traits.

The expression of many genes remains completely inhibited in most cell types. The DNA in human cells includes over 20,000 genes, but only a subset of these genes are transcribed in any particular cell type. Different cell types express different sets of genes, meaning that they transcribe some genes into mRNA, and not others. Different cell types have very different sets of proteins in them, causing them them to look and function very differently. Furthermore, cells regularly turn gene expression up or down, depending on cellular needs and environmental signals.