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 inherit DNA from all of those third-great-grandparents.
We also share similar percentages among other relatives. For example, full siblings will also share roughly 50 percent of their atDNA because they inherit from the same parents.
However, because DNA inheritance is totally random, siblings will not inherit exactly the same DNA from each parent. That means that I may have inherited DNA from one of those third-great-grandparents that my sister did not, and vice versa.
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, based on a lot of research:
It 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.
So, because of inheritance, DNA testing can be used even if the person on the other end of a relationship you want to prove is deceased. Even if they've been deceased for decades.
We use "reverse genealogy" to find living relatives to test and prove or disprove relationship theories. This requires building a tree back to a common ancestral couple and then building it forward to determine all of that couple's children (as opposed to simply the direct ancestor). Then we research to determine everyone in the next generation, and so on, until we get to the present and can find these living relatives.
More questions? Send them to me at TwistedRootsGenealogy@gmail.com, and I'll do my best to find the answers you need.