You don’t share the same amount of DNA with your relatives. To understand why, let’s review how genetic inheritance works.
You get half of your DNA from each parent and only half of their DNA. However, which half is passed down is random. So, while you and your sibling each got half of your DNA from your mom and half from your dad, only some of your DNA is the same. And while you and a cousin each inherited roughly 25 percent of your DNA from each of your shared grandparents, even less of that DNA is the same. The more distantly related you are to someone, the less DNA you share (Figure 1). You even have distant cousins with whom you don’t share any DNA. That's why if you want to discover distant branches of your family tree, having other family members test with AncestryDNA is helpful.
Figure 1. Graphic showing the relative amounts of DNA individuals of the indicated relationships share.
Find individuals with known relationships
The first step in estimating relationships is to understand how much DNA is shared by people of different relationships. To do this, we used a dataset of individuals whose relationships had been identified—and we were confident that the relationships were correct. Using this data, for each of these relationships, we then counted the number of "inheritance events" or steps between the two people (Figure 2).
Figure 2. Inheritance events. A. You and your sibling have two inheritance events separating the two of you: one step from you to your father and another step back down to your sibling. B. In this example, you and your cousin are separated by eight inheritance events: five steps from you to your common ancestor and three steps back down to your cousin.
See how much DNA is shared in different relationships
Once we have a large set of known relationships, we can look at the amount of DNA shared by two people in a given relationship. For example, we looked at many people who were first cousins (who have four inheritance events) and found a range in the amount of DNA they share. We created a distribution chart showing how much DNA is shared between individuals with different inheritance events (Figure 3).
Figure 3. Amount of DNA shared (in centimorgans) between people with the indicated relationships.
Estimate a relationship range
The distribution chart shows the expected amount of DNA shared between people who are separated by a given number of inheritance events. So, for you and your DNA match, we look at how much DNA you share and translate that to the most likely number of inheritance events separating you using the distribution chart. It's fairly easy to determine a relationship type for a pair of individuals with only one inheritance event (parent/child) or even two inheritance events (siblings) apart because the amount of DNA shared in these relationships is distinct from other relationships.
However, the randomness of DNA inheritance means we can’t always determine the exact relationship between AncestryDNA members. For example, the amount of DNA shared by 3rd and 4th cousins is very similar—meaning it’s harder to tease apart these relationships with certainty. It’s even more difficult to tell the difference between 5th and 8th cousins because the amount of DNA shared between these distant relationships is highly variable.
We'll predict the relationship as one of our match categories based on the amount of DNA you share with a match. Several types of relationships may fall into a single match category.
If you want to learn more details about this process, including how we find matches and predict their relationship to you, check out our white paper on DNA matching.