Bob Jenkins has published an interesting article concerning the origins of your DNA. As he writes:
“The number of genealogical ancestors you have n generations is 2n: 2 parents, 4 grandparents, 8 great-grandparents, and so forth. The only way to have fewer is if some of them are the same person. (For example, I have two great-great-great-great grandparents who are also my great-great-great-great-great grandparents by another line.)
“The difference between genealogical ancestors and genetic ancestors is that genetic ancestors are the ones that you actually got some DNA from. They’re a subset of your genealogical ancestors. Humans have about 3 billion base-pairs of DNA, so that limits the number of genetic ancestors to about 3 billion no matter how far back you go. There are also around 46,000 hotspots (reference Genetic Crossovers Are Predicted Accurately by the Computed Human Recombination Map, figure 6). Hotspots are the places where crossovers usually happen. Each of the 46,000 segments bounded by neighboring hotspots usually has a single line of descent, so you’re limited to about 46,000 ancestors.
“But I’m interested in something that happens much sooner than that: after about eight generations back, the number of genetic ancestors only increases linearly with the number of generations, while the number of genealogical ancestors keeps increasing exponentially. Once you go back 20 generations, you have only 1300 or so genetic ancestors despite having over a million genealogical ancestors.”
He also writes:
“One practical upshot of this is that those ancestors you do inherit something from, you inherit a lot from. And large stretches of DNA are passed down from generation to generation, those stretches usually aren’t finely divided.”
The article includes tables showing the “crossovers.” You can read Bob Jenkins’ article in his personal web site at: http://burtleburtle.net/bob/future/ancestors.html.