Final answer:
Gene expression patterns in differentiated cells are primarily determined by epigenetic changes, such as DNA methylation and histone modifications, which regulate selective gene expression and cellular specialization.
Step-by-step explanation:
The gene expression pattern for any type of differentiated cell is determined primarily by epigenetic changes. These are heritable changes in gene expression that do not involve alterations to the genetic code itself. Instead, they frequently involve modifications to chromatin structure, such as DNA methylation and histone modification, which alter how DNA is packaged within the nucleus and how accessible it is to the cellular machinery that governs transcription.
Within a multicellular organism, while every cell contains the same DNA, not every gene is expressed in each cell. The process of differentiation, where a cell becomes specialized for a certain function, involves 'reading' only the portions of DNA relevant to its function. This selective expression determines the cell's properties and tasks, akin to how actors in a movie only perform their part of the script, despite having access to the entire script. Furthermore, environmental factors can also influence gene expression patterns, further tailoring the functional profile of a cell.