Certain genes that allow for a certain thing to happen are turned on or turned off as needed. A brain cell may have the gene needed to send electrical pulses to other brain cells or other parts of the body, whereas a heart cell may have the genes that allow it to contract to pump blood. (I don't know if those are real genes, but hopefully you get the idea)
The DNA of heart cell and the DNA of brain cell is same. Their function is also same. They have genes that function the same.
The only difference is the functional genes. Different regions of the genes are functional in heart cell and different region is active in brain cells.
The genes in the heart makes different types of cells and tissues that helps in pumping the blood to the different parts of the body.
The cells in the brain gives rise to neurons and other brain cells which helps in processing of the information.
