All zygotes of human have the same genotype.

But, when you consider carefully, starting from the unique cells, each cells divided should have the same structure, what causes them to specialize to carry out different functions?

There is no simple answer because there are several ways that it happens. Here's a link to part of a textbook.

 

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=morphogen+AND+mboc4%5Bbook%5D+AND+374514%5Buid%5D&rid=mboc4.section.3792#3814

maybe DNA causes cells to become specialized:rolleyes:

um yes and no... genes are a part of it, but the environment is also responsible. Fetuses can't be developed properly unless they are in the proper womb (which acts as the environment), therefore once again its a nurture vs. nature debate.

 

Post Scriptum: MBoC is a blast to read. You seriously can't get any more monotonous

The short, short answer: The DNA is always the same, but only a subset it expressed, different in each cell type.

look up embryology, and specifically, eve, hunchback, bicoid and nanos genes responsible for segmentation, it'll clarify some things for you

"HOX" and "PAX" in google should pull up some good stuff too.