One of the most common questions I receive, especially in light of the interest in ethnicity testing, is how much of an ancestor’s DNA someone “should” share.
The chart above shows how much of a particular generation of ancestors’ DNA you would inherit if each generation between you and that ancestor inherited exactly 50% of that ancestor’s DNA from their parent. This means, on the average, you will carry less than 1% of each of your 5 times great-grandparents DNA, shown in generation 7, in total. You’ll carry about 1.56% of each of your 4 times great-grandparents, your 6th generation ancestors, and so forth.
As you can see, if you’re looking for a Native American ancestor, for example, who is 7 generations back in your tree, if you carry the average amount of DNA from that ancestor, it will be less than 1% which will be under the noise threshold for detection – and that’s assuming they were 100% Native at that time.
Everyone inherits 50% of their DNA from their parents, but not everyone inherits half of each of their ancestors’ DNA from a parent. Sometimes, the child will inherit all of a segment of DNA from an ancestor, and in other cases, the child will inherit none. In some cases, they will inherit half or a portion of the DNA from an ancestor. In reality, the DNA segments are very seldom divided exactly in half, but all we can deal with are averages when discussing how much DNA you “should” receive from an ancestor, based on where they are in your tree.
The generational relationship chart above represents the average that you will inherit from each of those ancestors. Of course, few people are actually average, and you may not be either. In other words, your ancestor’s DNA may not be detectible at 5, 6 or 7 generations, because it was lost in generations between them and you, while another ancestor’s DNA is still present in detectable amounts at 8 or 9 generations.
How Does Inheritance of Ancestral Segments Actually Work?
For you to inherit a particular segment from one GGGGG-grandparent, the inheritance might look something like this. “You” are at the bottom of the tree. You can click on any graphic to enlarge.
In the above example, you inherited one-tenth of the segment from your GGGGG-grandparent which was one-third of the DNA that your parent carried in that segment from that ancestor.
A second example is every bit as likely, shown below.
In this second scenario, you inherited nothing of that segment from your GGGGG-grandparent.
A third scenario is also a possibility.
In this third scenario, you inherited all of the DNA from that ancestor as your parent.
Now, think of these three scenarios as three different siblings inheriting from the same parent, and you’ll understand why siblings carry different amounts of DNA from their ancestors.
Of course, the child can only inherit what the parent has inherited from that ancestor, and if that particular segment was gone in the parent’s generation, or generations before the parent, the child certainly can’t inherit the segment. There is no such thing as “skipping generations.”
In this fourth scenario, the parent didn’t receive any of the segment from the GGGGG-grandparent, but maybe their brother or sister did, which is why you want to test aunts and uncles. Testing everyone in your family available from the oldest generation is absolutely critical.
This, of course, is exactly why we test as many relatives as we can. Everyone inherits different amounts of segments of DNA from our common ancestors. This is also why we map our matching segments to those ancestors by triangulating with cousins – to identify which pieces of our DNA came from which ancestor.
Seeing examples of how inheritance works helps us understand that there is no “one answer” to the question we want to know about each ancestor – “How much of you is in me?” The answer is, “it depends” and the actual amount would be different for every ancestor except your parents, where the answer is always 50%.
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