[jamest (OP)]

Jon writes in: 'recently, someone said that there are 400 cell types in the human body. How are these cell types different at the genetic level?'

[paul cotter]

Apart from red blood cells and platelets which have no genetic material, all cells carry identical genetics. A slight complication occurs with reproductive cells, ie eggs and sperm cells which have only one set of chromosomes whereas all other cells that carry genetic information have a double set. What differentiates cell function is what genes are switched on/off and not what genes it contains as all cells carry all the info to produce any tissue. A stem cell refers to a cell that has not yet been programmed and has the capability to become any one of the many dedicated cells within the body.