Epigenetics plays a significant role downstream of the raw genetic inherited compliment (genotype). This often leads to a varied gene expression (phenotype) resulting from an identical genotype, and the converse as your question states. Examples of downstream (btw, 'downstream' here means after the individual has been conceived, and is in the early stages of embryonic development) phenomena that can alter the phenotype of a given gene are DNA methylation and protein repression. For much more detail, read this article.

http://en.wikipedia.org/wiki/Epigenetics