[Curious Ash (OP)]

If all cells in the same organism all contain identical genes then what tells each cell which genes to turn on and which to turn off to make them into different types of cells?

[RD]

Chemical concentration gradients can determine which cell type is created where in the embryo...

Slowly an important idea emerged: the gradient hypothesis. One of the proposers of this idea was Theodor H. Boveri of the University of Würzburg, the founder of the chromosomal theory of inheritance. Boveri suggested that "a something increases or decreases in concentration" from one end of an egg to the other. The hypothesis, in essence, is that cells in a developing field respond to a special substance - a morphogen - the concentration of which gradually increases in a certain direction, forming a gradient. Different concentrations of the morphogen were postulated to cause different responses in cells.

http://www.eb.tuebingen.mpg.de/departments/3-genetics/christiane-nusslein-volhard/gradients-that-organize-embryo-development

[Curious Ash (OP)]

That is so cool. Thanks