When a child or any organism develops from beginning stages of life, individual genes activate and “lock up” portions of a genome with hi stones or other DNA folding. This makes certain cells specialize, since all cells except for gametes in the human body have the same DNA, but it can be “turned off” to make layers of cells called tissues. In humans the study of this is called embryology.
Only multicellular organism make tissues and specialize each cell. The gene is the smallest functional unit in the DNA code. Some genes combine to make more complex traits such as eye color or blood type.
