Before we begin, an important correction: Each gamete (sperm and egg cell) contain HALF of the parents DNA.

Imagine you have two books, well call them Mom and Dad. Lets say for this example that each book has 23 pages.

You want to create a new book from those two books. To make the new book for each page you take half the text from the mom book for the same page, and half the text from the dad book for each page. Lets simplify it even more and say you take either the top half or the bottom half from each book.

So for any child page there are four possible combinations:

- Top Mom + Top Dad

- Top Mom + Bottom Dad

- Bottom Mom + Top Dad

- Bottom Mom + Bottom Dad

How many possible combinations of a 20 page book can you create from those four patterns for each page? A lot. 4^23 is over 70 TRILLION possible combinations. So you have a 1 in 70 TRILLION chance of making an exact duplicate using this method of making "new" books.

This is a VERY simplified version of how genetics works and already you can see how with even just a few options it becomes basically impossible to have identical copies from repeating the same process.