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How Far Back is Your 100% Ancestor from your DNA Results?

Are you hoping to find the ancestor from whom you inherited a percentage on your DNA ethnicity estimate? In this post, learn how to know how far back your “100% ancestor” might be in your family tree.

How Far Back is Your 100% Ancestor

There are many reasons that people are interested in understanding how many generations back their “100% ancestor” might be. Most commonly, people would like to know exactly where in their family tree they should look to find the person who passed that ethnicity region down to them.

When I first took a DNA test, I was interested in finding out how many generations back I would need to look to know who my 3% European Jewish DNA came from. My husband shows that 1% of his DNA corresponds with Senegal, Cyprus, and Sweden and Denmark.

Other people might be curious about larger percentages. For example, some people might show 50%, 75%, or even 100% of their DNA matching one region, but their parents were born elsewhere.

Estimate by doubling the percentage each generation

We are taught to understand the general pattern of ethnicity inheritance to be that 50% of each region is passed down to the next generation of offspring. In theory, this would mean that someone showing 100% of their ancestry from Ireland would have children and grandchildren who show 50% and 25% of their DNA matching Ireland, respectively.

Using this basic idea, we can estimate the following percentages for people descended from a person who would have shown 100% of their ethnicity matching a single region on a DNA test:

  • great-grandchildren will share 12.5% from the region
  • great-great grandchildren will share 6.25% from the region
  • great-great-great grandchildren will share 3.125% from the region
  • great-great-great-great grandchildren will share 1.5% their DNA from the region
  • great-great-great-great-great grandchildren will share no detectable DNA from the region

If you have tested your DNA and you have also worked a lot on your family tree, you will immediately notice that the estimates don’t seem to be 100% accurate. If you picked up on this, you are correct.

For example, my 3% European Jewish DNA could lead me to believe that I have a great-great-great grandparent who was 100% European Jewish. Based on my family tree that I have verified through DNA matches, none of my great-great-great grandparents identified as Jewish.

So, what’s going on here?

The 50% estimate is only a rule of thumb

While the estimates I mentioned above are a good place to start, they are only a basic guideline for estimating how far back the ancestor might be.

They are most accurate when the percentages are higher (i.e. you suspect your “100% ancestor” could be a grandparent or great-grandparent), and when you believe that your DNA from that region was inherited from a single person and not more than one ancestor on multiple lines of your tree.

DNA is passed down to children in a random process called recombination. This is where two copies of a chromosome “recombine” to make a brand-new chromosome that contains pieces from both copies of the parent’s chromosome.

The process of recombination leaves 50% of the parent’s DNA behind because our chromosomes are a set size and can’t contain all of our parents’ DNA. As we move down in the generations away from the “100% ancestor” we will gradually find less of that person’s DNA in the chromosomes of their descendants.

To visualize how this works in an overly simplified example, imagine a bag of 100 blue marbles as the DNA of my mysterious Jewish ancestor, who we will imagine is a woman. When she produced a child, that child now had a bag with only 50 blue marbles. The other 50 were other colors representing the father’s ethnicities (let’s say 25 red marbles and 25 green).

When the child had a child, a random selection of her marbles were chosen to pass down. Imagine reaching into this bag containing 50 blue marbles, 25 red marbles, and 25 green marbles with your eyes closed and picking out 100 marbles.

Chances are very high that you would get a lot of blue marbles, but probably not exactly 25 (i.e. 50% of the Jewish ancestry). You would get several red and green marbles, too.

Now, when the grandchild of my Jewish ancestor had a child, the same thing would happen. However, now the bag of marbles has many fewer blue marbles – maybe even as few as 20.

As the generations continue to pass, the likelihood of the blue marbles being chosen randomly to be passed down decrease. Furthermore, since it happens at random, it is not an even 50% decrease every generation.

We can inherit more than 50% of an ethnicity region from our parents, or less. Or, we can inherit 100% of their DNA matching one region, especially if it is a small percentage.

Small percentages can be passed down for several generations before disappearing

If you inherited a small percentage of DNA, you should know that it is possible for small percentages of DNA to be passed down for multiple generations. This is because we might have inherited entire DNA segments matching the region from our parent, who inherited the same unbroken DNA segment from their parent.

This is one reason why our marble example is too simplified. It would be better, for this section of the article, to imagine that our marbles are strung together in short strands.

If you have 1% of your DNA matching a specific region, your parent could have also had 1 or 2%. Your grandparent may have only had 1-2% matching that region, too.

You may have simply inherited the entire strand of red marbles.

It is important to consider this in researching your ancestor. This would mean that they are further back in your tree, so instead of looking for a 4th great-grandparent, you might be looking for a 7th great-grandparent, or even someone more distant.

I’ll continue with my search for my Jewish ancestor as an example. I showed 3% on my results, and my mother showed only 4%.

This means that I essentially inherited all of my mother’s DNA matching the Jewish ethnicity region. My mother has a few siblings who have also tested their DNA with the same company, and their results show between 1-5% Jewish.

Because of the Ancestry Sideview Ethnicity Inheritance tool, I know that my mother inherited her Jewish DNA from her mother. And, because one of her children only shows 1% Jewish, I feel like my mother and her other siblings may have inherited all of my grandmother’s Jewish DNA – another clue that the ancestor is likely further back than the estimated 4th great-grandparent.

A high percentage could indicate a very recent ancestor

If you have a high percentage, such as 30-50%, you could be looking for a very recent ancestor. A parent or a grandparent could be a possibility. This is because we typically inherit 50% of our parents’ DNA regions.

If your parent had 100%, or close to 100%, of one specific ethnicity (if they were available to test their DNA) we would expect you to show a percentage close to 50% matching that same region.

Or, many more distant ancestors

It is also possible to inherit DNA from one or both of your parents matching the same ethnicity region. This can make it more difficult to estimate where you should look to find ancestors from, or born in, the region.

For example, if both your mother’s father and your father’s father had ancestry from Senegal, you might show a relatively large percentage of your DNA matching this region on your results.

However, you would have to look further back in your family tree on both lines in other to find your Senegalese ancestor.

Both of your parents may have had the same ethnicity

For example, my grandmother shows 79% of her DNA matching England and Northwestern Europe and my husband shows 94% of his DNA as the Indigenous Americas region. This means that both my husband and my grandmother had ancestry from those regions on both sides of their family.

Essentially, almost all of my grandmother’s ancestors on both of her parents side were of English ancestry. Since several generations of her ancestors were born in the United States, we would need to look very far back in her tree to find ancestors born in England.

In my husband’s case, his 94% Native American ancestry means that both of his parents would have shown very close to 100% of their ancestry matching Indigenous Americas, if they were available to take a DNA test. In his case, his parents also identified as indigenous, so he doesn’t have to look far to find his “100% ancestors”.

The beauty of family tree research, and DNA results, is that every single person has a unique family story. Researching how our ancestors passed their DNA down to us always adds to the uniqueness of our story.

Your ancestor from the ethnicity region might not have been “100%”

When we are searching for a specific ancestor who may have passed a specific region down to us, we should remember that many of our ancestors may have had as diverse ancestry as we do. This means that some of our ancestors born in a specific place may not have shown up as “100% matching that region on a DNA test.

Our ancestors traveled more than we often think they could have, despite the difficulties they faced at the time.

So, when you are researching your family tree, keep in mind that your Italian, Polish, or Russian ancestor, if they could take a DNA test today, may not have shown up as “100%”. This is actually good news – we may not always have to look as far back as we think for some of our more elusive DNA regions.


I hope that this article has helped you understand all of the considerations that you should take into account before beginning your research into your ancestor who may have had 100% of their ethnicity from the region on your DNA results.

If you have any questions about something that you read here, or if you would like to ask a specific question about a region on your results that you are curious about, I encourage you to leave a comment in the discussion below.

Thanks for reading today!

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