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Concepts – Percentage of Ancestors’ DNA

Roberta Estes

A very common question is, “How much DNA of an ancestor do I carry and how does that affect my ethnicity results?”

This question is particularly relevant for people who are seeking evidence of a particular ethnicity of an ancestor several generations back in time. I see this issue raise its head consistently when people take an ethnicity test and expect that their “full blood” Native American great-great-grandmother will show up in their results.

Let’s take a look at how DNA inheritance works – and why they might – or might not find the Native DNA they seek, assuming that great-great-grandma actually was Native.

Inheritance

Every child inherits exactly 50% of their autosomal DNA from each parent (except for the X chromosome in males.) However, and this is a really important however, the child does NOT inherit exactly half of the DNA of each ancestor who lived before the parents. How can this be, you ask?

Let’s step through this logically.

The number of ancestors you have doubles in each generation, going back in time.

This chart provides a summary of how many ancestors you have in each generation, an approximate year they were born using a 25 year generation and a 30 year generation, respectively, and how much of their DNA, on average, you could expect to carry, today. You’ll notice that by the time you’re in the 7th generation, you can be expected, on average, to carry 0.78% meaning less than 1% of that GGGGG-grandparent’s DNA.

Looking at the chart, you can see that you reach the 1% level at about the 6th generation with an ancestor probably born in the late 1700s or early 1800s.

It’s also worth noting here that generations can be counted differently. In some instances, you are counted as generation one, so your GGGGG-grandparent would be generation 8.

In general, DNA showing ethnicity below about 5% is viewed as somewhat questionable and below 2% is often considered to be “noise.” Clearly, that isn’t always the case, especially if you are dealing with continental level breakdowns, as opposed to within Europe, for example. Intra-continental (regional) ethnicity breakdowns are particularly difficult and unreliable, but continental level differences are easier to discern and are considered to be more reliable, comparatively.

If you want to learn more about how ethnicity calculations are derived and what they mean, please read the article Ethnicity Testing – A Conundrum.

On Average May Not Mean You

On average, each child receives half of the DNA of each ancestor from their parent.

The words “on average” are crucial to this discussion, because the average assumes that in fact each generation between your GGGGG-grandmother and you inherited exactly half of the DNA in each generation from their parent that was contributed by that GGGGG-grandmother.

Unfortunately, while averages are all that we have to work with, that’s not always how ancestral DNA is passed in each generation.

Let’s say that your GGGGG-grandmother was indeed full Native, meaning no admixture at all.

You can click to enlarge images.

Using the chart above, you can see that your GGGGG-grandmother was full native on all 20 “pieces” or segments of DNA used for this illustration. Those segments are colored red. The other 10 segments, with no color, were contributed by the father.

Let’s say she married a person who was not Native, and in every generation since, there were no additional Native ancestors.

Her child, generation 6, inherited exactly 50% of her DNA, shown in red – meaning 10 segments..

Generation 5, her grandchild, inherited exactly half of her DNA that was carried by the parent, shown in red – meaning 5 segments..

However, in the next generation, generation 4, that child inherited more than half of the Native DNA from their parent. They inherited half of their parent’s DNA, but the half that was randomly received included 3 Native segments out of a possible 5 Native segments that the parent carried.

In generation 3, that child inherited 2 of the possible 3 segments that their parent carried.

In generation 2, that person inherited all of the Native DNA that their parent carried.

In generation 1, your parent inherited half of the DNA that their parent carried, meaning one of 2 segments of Native DNA carried by your grandparent.

And you will either receive all of that one segment, part of that one segment, or none of that one segment.

In the case of our example, you did not inherit that segment, which is why you show no Native admixture, even though your GGGGG-grandmother was indeed fully Native..

Of course, even if you had inherited that Native segment, and that segment isn’t something the population reference models recognize as “Native,” you still won’t show as carrying any Native at all. It could also be that if you had inherited the red segment, it would have been too small and been interpreted as noise.

The “Received” column at the right shows how much of the ancestral DNA the current generation received from their parent.

The “% of Original” column shows how the percentage of GGGGG-grandmother’s DNA is reduced in each generation.

The “Expected” column shows how much DNA, “on average” we would expect to see in each generation, as compared to the “% of Original” which is how much they actually carry.

I intentionally made the chart, above, reflect a scenario close to what we could expect, on average. However, it’s certainly within the realm of possibility to see something like the following scenario, as well.

In the second example, above, neither you nor your parent or grandparent inherited any of the Native segments.

It’s also possible to see a third example, below, where 4 generations in a row, including you, inherited the full amount of Native DNA segments carried by the GG-grandparent.

Testing Other Relatives

Every child of every couple inherits different DNA from their parents. The 50% of their parents’ DNA that they inherit is not all the same. The three example charts above could easily represent three children of the GG-Grandparent and their descendants.

The pedigree chart below shows the three different examples, above.  The great-great-grandparent in the 4th generation who inherited 3 Native DNA segments is shown first, then the inheritance of the Native segments through all 3 children to the current generation.

Therefore, you may not have inherited the red segment of GGGGG-grandmother’s Native DNA, but your sibling might, or vice versa. As you can see in the chart above, one of your third cousins received 3 native segments from GGGGG-grandmother. but your other third cousin received none.

You can see why people are always encouraged to test their parents and grandparents as well as siblings. You never know where your ancestor’s DNA will turn up, and each person will carry a different amount, and different segments of DNA from your common ancestors.

In other words, your great-aunt and great-uncle’s DNA is every bit as important to you as your own grandparent’s DNA – so test everyone in older generations while you can, and their children if they are no longer available.

Back to Great-Great-Grandma

Going back to great-great-grandma and her Native heritage. You may not show Native ethnicity when you expected to see Native, but you may have other resources and recourses. Don’t give up!

Reason Resources and Comments
She really wasn’t Native. Genealogical research will help and mitochondrial DNA testing of an appropriate descendant will point the way to her true ethnic heritage, at least on her mother’s side.
She was Native, but the ethnicity test doesn’t show that I am. Test relatives and find someone descended from her through all females to take a mitochondrial test. The mitochondrial test will answer the question for her matrilineal line unquestionably.
She was partly, but not fully Native. This would mean that she had less Native DNA than you thought, which would mean the percentage coming to you is lower on average than anticipated. Mitochondrial DNA testing someone descended from her through all females to the current generation, which can be male, would reveal whether her mother was Native from her mother’s line.
She was Native, but several generations back in time. You or your siblings may show small percentages of Native or other locations considered to be a component of Native admixture in the absence of any other logical explanation for their presence, such as Siberian or Eastern Asian.

Using Y and Mitochondrial DNA Testing to Supplement Ethnicity Testing

When in doubt about ethnicity results, find an appropriately descended person to take a Y DNA test (males only, for direct paternal lineage) or a mitochondrial DNA test, for direct matrilineal results. These tests will yield haplogroup information and haplogroups are associated with specific world regions and ethnicities, providing a more definitive answer regarding the heritage of that specific line.

Y DNA reflects the direct male line, shown in blue above, and mitochondrial DNA reflects the direct matrilineal line, shown in red. Only males carry Y DNA, but both genders carry mitochondrial DNA.

For a short article about the different kinds of DNA and how they can help genealogists, please read 4 Kinds of DNA for Genetic Genealogy.

Ethnicity testing is available from any of the 3 major vendors, meaning Family Tree DNA, Ancestry or 23andMe. Base haplogroups are provided with 23andMe results, but detailed testing for Y and mitochondrial DNA is only available from Family Tree DNA.

To read about the difference between the two types of testing utilized for deriving haplogroups between 23andMe and Family Tree DNA, please read Haplogroup Comparisons between Family Tree DNA and 23andMe.

For more information on haplogroups, please read What is a Haplogroup?

For a discussion about testing family members, please read Concepts – Why DNA Testing the Oldest Family Members is Critically Important.

If you’d like to read a more detailed explanation of how inheritance works, please read Concepts – How Your Autosomal DNA Identifies Your Ancestors.

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