With the advent of the internet, the ready access to a plethora of genealogically relevant materials is increasing the possibility of tracing our ancestral lines back into the early 18th, 17th, and even 16th centuries.

All too frequently, however, I see people focusing on laying claim to a specific ancestral group and identifying with that group only. For example, let’s say someone is descended, by way of their grandfather’s great-grandmother’s father’s patriline, from some famous (or infamous) Duke or military hero. If this individual lived 10 to 12 generations ago, he may not be as significant (but still interesting) in the researcher’s genetic ancestry as they believe. Let me explain.

We can, in a perfect world, mathematically describe the number of ancestors we have with the equation (Npeople)=(2^Nancestral_generation). So, if we were to look at our great-grandparents, we have 8 ancestors at that level because we have 2^4 ancestors (we can mathematically refer to ourselves as generation 0 since 1(self)=2^0. Make sense? I hope you can see how the easily scales up rapidly.

At 10 generations ago, 2^10 = 1024. Let that sink in for a moment. It took 1024 people who lived 10 generations ago to bring you into existance. That’s pretty wild.

I hope that when delving into genealogical research, one does not become preoccupied with a distant specific family or individual. Even when researching your father’s 10th-great-grandfather, there are still 4098 individuals from that ancestral population that you are descended from (barring any intermarriage of cousins).

An interesting thing happens, however, once we start going very far back in time. Let’s say we can trace a line back to The Norman Conquest. This should be around 40 generations. This number 2^40 = 1,099,511,627,776 (1 trillion, 99 billion, 511 million, 627 thousand, 766). Now, we know (and we do know this) that the human population 40 generations ago was not 1 trillion people. Here’s a great article that explains it all much more thoroughly than I can.

Ok, so now that we understand the scale of things, lets look at how it relates to DNA. The ISOGG states that a parent and child will share about 3400cM (centiMorgans), and I can confirm this from my DNA results. Luckily, this roughly scales linearly when looking at the number of generations ago we descend from someone.

For example, we can suggest that we share 1700cM with a grandparent, 850cM with a great-grandparent, and 425cM with a 2x-g-grandparent, and so on. Further, this can be used to estimate our expected relationship distance with a DNA match.

Here’s where things get complicated. Let’s say two individuals are descended from the same 4th-great-grandfather and 4th-great-grandmother. The numbers suggest that both individuals should have inherited about 106.25cM of DNA from each of these individuals. This does not mean that both of the individuals who have tested their DNA will share all of their 106.25cM with the other descendant. This number, 106.25cM, is a max theoretical value and depends on i) which DNA segment they inherited ii) if they inherited all of the same DNA segment, and iii) if the DNA segment was not divided into smaller noncontiguous segments. It is not impossible for 5th cousins to share 106.25cM, but it is usually the exception rather than the rule requiring both individuals to have inherited the exact same segment of DNA. Additionally, this number can be affected by whether or not cousin marriages occurred, an event that may increase this number.

So, to wrap things all together, we can suggest that from an ancestor of ours who lived 20 generations ago, we inherited ~0.0065cM of DNA from them, theoretically. This translates to rougly 6484 base pairs. One centiMorgan is roughly 1 million base pairs, so we inherited very little DNA (6484/1,000,000 base pairs) from an ancestor 20 generations ago. Again, this value can vary based upon the intermarriage of cousins.

Now yDNA, that’s a whole different story.

Thanks for reading.

Chris