Visit this post on my site: The Ghosts in Our Genomes: Why We Don’t Have DNA From Most of Our Ancestors

As we look at our family trees, stretching back centuries, we might feel a connection to the names and want to understand the stories of our ancestors. Assuming accuracy of the research, we are related to every person on our family trees. But our DNA tells a different, more selective story where most of those ancestors are, genetically speaking, complete strangers.

We might see something like this if we took an ancestral DNA test. We might have expected to see the Italian ancestry from a 4th-great-grandmother, but it’s not there. Was there a family secret? Not necessarily. The truth lies in the imperfection of DNA testing but also in the mathematics of genetic inheritance, which is the focus of this article.

##### The 50/50 Split: A Shuffle Every Time

It all starts simply enough. We inherit 50% of our DNA from our mothers and 50% from our fathers. This genetic information is contained in the autosomal chromosomes (pairs 1-22).

Parents’ DNA can be thought as two full decks of cards. To make each new deck they each shuffled their deck and handed a child half of their cards. The child’s genome is this new, unique deck. This process of shuffling (called recombination) and passing on only half is repeated every single generation. That might seem like someone will end up with some DNA from all ancestors, even a minuscule amount, but there is randomness to genetics. This randomness has profound consequences.

##### The Disappearing Ancestor

Because each person get half of their DNA from each parent, the genetic contribution of any single ancestor gets diluted very quickly. With each generation back, the amount of DNA from a specific ancestor is halved.

Let’s look at how rapidly this happens:

| Generation | Ancestor Title | Number of Potential Ancestors | Average DNA Contribution |
|————|———————–|——————————–|—————————|
| 1 | Parent | 2 | 50% |
| 2 | Grandparent | 4 | 25% |
| 3 | Great-Grandparent | 8 | 12.5% |
| 4 | 2nd Great-Grandparent | 16 | 6.25% |
| 5 | 3rd Great-Grandparent | 32 | 3.125% |
| 6 | 4th Great-Grandparent | 64 | 1.563% |
| 7 | 5th Great-Grandparent | 128 | 0.781% |
| 8 | 6th Great-Grandparent | 256 | 0.391% |
| 9 | 7th Great-Grandparent | 512 | 0.195% |
| 10 | 8th Great-Grandparent | 1,024 | 0.098% |
| 11 | 9th Great-Grandparent | 2,048 | 0.049% |
| 12 | 10th Great-Grandparent| 4,096 | 0.024% |

By the time someone traces back to their 10th great-grandparent (12 generations back), less than 0.03% of their DNA, on average, comes from any individual in that generation. Because recombination is random, it quickly becomes possible—and eventually, probable—that people inherit zero DNA from a specific distant ancestor. Billions of people are on our family trees, but not a single gene from most ancestors made it into our personal deck of cards, as will be explained.

##### The Ancestor Explosion

This is where it gets truly mind-boggling. Let’s do some quick math. Each person has 2 parents, 4 grandparents, 8 great-grandparents, and so on. The number of potential ancestors doubles each generation.

What happens if we go back 50 generations, to around the year 800 AD? The number of theoretical ancestors someone has is 2^50, which is **over 1.1 quadrillion people**. That number presents a problem: it’s vastly more people than have ever lived on Earth.

The solution is a concept called **pedigree collapse**. Our family trees aren’t perfectly branching trees at all. As we go back in time, the branches start to fold back in on themselves as close or distant cousins had children together, which was almost unavoidable in smaller, more isolated populations. This results in all people having the same ancestors many, many times over in different branches of our trees.

##### Genealogical vs. Genetic Ancestors: A Distinction

This brings us to the core idea. We must distinguish between two types of ancestors:

1. **Genealogical Ancestors**: Every single person on our complete family tree.
2. **Genetic Ancestors**: The subset of those genealogical ancestors from whom we inherited a detectable segment of DNA.

While our genealogical trees branches out into the millions, those trees are constantly being pruned by the random 50/50 shuffle of inheritance.

Let’s go back to that 50th generation. Estimates of the world population in about 800 AD are somewhere **between 200 and 300 million people**; thus, it’s not possible to have a quadrillion ancestors about 1250 years ago. Due to pedigree collapse, a conservative estimate might be around **50 million unique genealogical ancestors** 50 generations back. But how many of our ancestors gave us DNA?

On average, we inherit (1/2)^50 or about 8.88×10^-16 or 0.0000000000000888% of our DNA from any given 50th generation ancestor. If this math is extrapolated out, genetic models estimate that we likely have DNA from only 100,000 to 200,000 of our 50th generation ancestors.

Think about that. Out of 50 million ancestors living around 1250 years ago, we carry the direct genetic legacy of, at most, 200,000. That is many people but it means **we have no traceable DNA from over 99.6%** of our ancestors from that era. They are ghosts in our genome. As we go further back, the percent of ancestors we share no DNA with approaches 100%.

##### What This Means for Us

This isn’t just a thought experiment; it has real-world implications.

– **DNA Tests**: It explains why siblings can have different ethnicity estimates. Siblings both inherit different, randomly shuffled decks of cards from their parents. Theoretically, it’s even possible, but extremely unlikely, for siblings to share no DNA! It also explains why that Italian 4th-great-grandmother might not show up—the tiny segment of DNA someone inherited from her may have been lost in the shuffles over even a few generation.
– **Famous Ancestors**: As an example, most or all people with any European ancestry should be able to trace their family tree back to Charlemagne (who lived about 50 generations or 1250 years ago). That does not mean family trees showing that connection are correct, however, because there are too many missing records to make the genealogies accurate for most or even all people. However, genealogically, this connection should exist for essentially all people alive today who have any European ancestry. But the chance that they have any of Charlemagne’s actual DNA is practically zero. Somone might be his genealogical descendant, but is unlikely to carry his genes.
– **Our Shared Humanity**: This process of pedigree collapse and the genetic lottery means we are all profoundly interconnected. If we go back enough generations, everyone alive today shares a common ancestor. That’s maybe as recently as 2,000 years ago. What does that mean? It means that within the last 3,000 years it’s highly likely a set of parents lived who are the ancestors or all people alive today. That makes us all distant cousins, sharing a small pool of genetic ancestors who passed their legacy on to the present day.

So appreciate all your ancestors but remember that we are the result of an epic genetic lottery, a unique combination of DNA passed down through a very select few.

Visit my blog: By Study and Faith