Once the female egg has been fertilized, the cells formed after cell division contain DNA that is identical. That is, the DNA in all the cells will be identical. However, different regions of a chromosome (DNA is wound in to a chromosome) code for different functions and cell type. Here, it’s only the regions that are required to perform a given function that are expressed in each cell.
The regions (genes) that are expressed determine the type of cell that will be created. While the different types of cells that are formed contain the same DNA, it’s the expression of different genes that results in different types of cells. This is to say that not all genes are expressed during differentiation.
* Gene expression is the process through which information from a given gene is used to develop the structures of specific cells.