(Tried Googling this - didn’t have much luck.)
As I understand it, nearly every cell contains a full copy of the organism’s DNA. But the cell’s development is influenced by a tiny part of that DNA - perhaps a single gene (yes?).
So how does this work?
To be very very brief:
There are tens of thousands of genes, and any of them can be active at the same time. Some genes will “turn on” or “turn off” other genes by changing whether those genes are transcribed into RNA or translated into proteins. Things are astoundingly complicated, and genes form vast interconnected regulatory networks.
I’m working on a PhD thesis that focuses on biochemical interactions between a small set of gene products in the fruit fly. If I’m lucky and clever, I’ll learn about a handful of the possible interactions and publish a few papers in the process.
ETA: For a more in-depth intro, Wiki’s article on gene expression is a decent place to start.
This wiki article on gene regulation actually seems better written for a non-expert and contains a lot of similar information.
Even that gets pretty complex. I think it is clear that there are many mechanisms involved in controlling which genes are expressed when based on what is around the cell and what is already in the cell at the time … but all that complexity only begs the question of how they started down those different paths in the first place.
The answer to that is the fertilized ovum is not a homogenous thing. There are gradients of mRNAs (and other regulatory RNAs) and proteins within the cytoplasm of the fertized ovum. As that cytoplasm gets divided into different cells those cells get different amounts of those maternally derived RNAs and proteins which direct each cell to express different combinations of genes. Typically the first thousand cells are produced by division of that cytoplasm directed by those maternally derived substances rather than new transcription and translation (reading of the new organism’s DNA and making new proteins). From there it is a cascade of control, response to the cells and substances around them, and producing substances that impact the regulation of cells around them.
No. It is true that any particular cell will only be expressing some of the genes its DNA contains at any one time, and some genes may never be expressed in certain cells, but any cell is going to make use of quite a large proportion of its DNA (of that part that actually contains genes, anyway - “junk” DNA may be another matter) and certainly much more than a single gene.
For the rest of your question, see what lazybratsche and DSeid have already said.