Zygotes (totipotent) contain all coding information needed for life within a single cell.
Through mitosis large anouts of cells replicate.
One gene will be switched on, producing a protein, called a transcription factor that cases a "gene cascade effect", which in turn causes other genes to be switched on, producing futher proteins.
Certain cells will have activated certain genes - not all, even though they are all totipotent.
To find out about the processes inovlved in gene cascade, scientists researched the drosophilia fly. They manipulated the DNA (recombo), finding that the material from certain genes seemed to cause developments of the structure components, other genes formed other segments between anterior and posterior components, for example the legs/arms. All these genes were termed HOM genes, and worked in a particular sequence, to form the structure of an organism.
A sequence of genes resposible for a particular structure (I think?) is termed a homeobox, or HOX.
