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Updated: Jan 19, 2020

Today I was asked a question from someone who is trying to prove a relationship within his family: "How can DNA tell me I'm related to someone if that person is dead and was never tested?"

That's really a great question, and here is a relatively short answer:

We inherit all of our DNA from our parents. First, there is autosomal DNA (atDNA), which makes up our 46 chromosomes. This is what AncestryDNA and 23andMe test specifically; the third largest company, FamilyTreeDNA, tests atDNA with it's "Family Finder" kit, but also offers Y-DNA and mitochondrial DNA (mtDNA).

But, we'll get to those another time. But if you have a burning desire to know more, please follow the links provided above or go directly to the International Society of Genetic Genealogists' website.

So, with atDNA, we each inherit roughly 50 percent of our chromosomes from mom and 50 percent from dad.

Because they also inherit half from each of their parents, they are actually passing down their ancestors' DNA.

So, for example, I get half of my mother's chromosomes; she got half from my grandfather and half from my grandmother. The same goes for dad. Therefore, I inherit roughly 25 percent of my DNA from each of those four grandparents; 12.5 percent from my eight great-grandparents, 6.25 percent from my 16 great-great-grandparents; and about 3 percent from my 32 great-great-great-grandparents.

Obviously, autosomal DNA is not infinite. Therefore, it is likely you will not inherit DNA from all of those third-great-grandparents. In fact, of all existing third cousins we have, statistically, we will not share DNA with about 25 percent of them; at the fourth-cousin level, statistically we will not share DNA with about 50 percent of them, even though, on paper, we are related. The fact is, atDNA inheritance is a very random thing.

Because of that randomness, even though full siblings share roughly 50 percent of their atDNA because they inherit from the same parents. But keep in mind, siblings do not inherit exactly the same DNA segments from each parent. That means that I may have inherited DNA from one of those third-great-grandparents and my sister did not, and vice versa. This highlights the usefulness of testing as many siblings and cousins as possible.

Aunts/uncles will share about 25 percent of their DNA with nieces/nephews; first cousins, who have the same grandparents, will share 12.5 percent, and so on.

Here is a chart created by the DNA Detectives:

This information is thanks to a great number of "citizen scientists" who have dedicated a lot of time, energy and talent to figuring out these percentages to create charts like this to help people decipher what it all means. In addition to charts, these folks have created online tools and have produced tons of research, all of which has brought us to where we are now.

And it's only just begun. There now is, which is offers a great tool that really illuminates the possible matches you would have with others based on the amount of shared DNA. Here is an example of the relationships that are most likely between people who share 937 cM:

By narrowing down a relationship with someone who matches your DNA, traditional research, then finding where they belong on your own family tree, atDNA can prove you are related to another person who hasn't or can't be tested. Because of inheritance, DNA testing can help prove a relationship to someone who is deceased, even if they've been deceased for decades.

To find further proof, I use "reverse genealogy" to find living relatives who may be enticed to take a DNA test. This requires building a tree forward to the present from a person or couple I hope to prove as ancestors. First, I determine as best I can all of that person's or couple's children, and then each child's descendants to get to living potential relatives. Then I use some of my investigative online skills to find contact information for the person I think would be best to contact about testing.

More questions? Send them to me at, and I'll do my best to find the answers you need.

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