Each of the cells in our body (with the exception of erythrocytes and thrombocytes) contains the genetic program of a human being. The genetic information is written down in chromosomes and stored in the nucleus, like in a treasury. The 46 human chromosomes can be compared to a library containing 46 books, in which the texts of our life is written. Our genetic program comprises of approximately 20.000 texts that are stored in books of different sizes. These texts are our genes. We carry two copies of each book, as we inherit one book from our mother and one book from our father. Two of these books determine our gender: women have all the books double (two X chromosomes) while men have one X chromosome and one Y chromosome.
All cells contain the same human library. The specialized cells of our body differ from their ability to read different texts. Texts that are not read in a certain cell, are sealed and cannot be accessed. The specialized cell types in our body, for example in nerves, bone, skin, kidneys or liver, are different because their ability to read different texts. During the human development the profile of the legible texts in specific cells changes. Therefore, a small mass of cells is able to turn into a fully developed human being during the embryonic development. At a particular time in our development a certain cell changes its reading pattern. This cells is subsequently the first cell of for example our skin or nervous system.