You have received your DNA results, and now you are going through your DNA relatives and keep on seeing something like a 1st – 2nd Cousin or 2nd – 4th Cousin. What does this mean in relation to you?
We have thousands of cousins, actually, billions of cousins. It is just that most of those cousins are very distantly related. But even when we are talking about close relationships, just about everyone has several hundred or more 4th cousins or closer.
When reporting DNA relationships, the companies don’t know exactly how you are related until you tell them. There are some relationships, like siblings and parents which are easy to figure out because of the amount of shared DNA. But relationships don’t have an exact amount of shared DNA (other than parents), but rather a range that is expected. Because of this, some relationships look like others, and the more distant the relationship, the more likely it can seem like something else when examining the amount of DNA.
To determine relationships, companies are looking at both the amount of shared DNA, and at the number of DNA segments shared. In general higher numbers of each, indicate a closer relationship. But there is still a range of relationships it could be because of the amount of overlap.
For instance, 2nd cousins share between 84 cM and 484 cM, and this may be over 10 to 35 segments.
Third cousins share between 0 cM and 174 cM, and that could be over 0 to 15 segments.
So you can see that there is part of each range of cM that overlaps, and part of each number of segments that overlap. When you get into half relationships and a generation removed relationships, there begins to be a lot of overlap.
So initially the companies will report a relationship as a range of possibilities, and it is up to you to figure out the specific possibility.
On a pedigree chart, second cousins share great-grandparents. Since everyone has 4 sets of great-grandparents, you can usually narrow down to a one or two as the possible common ancestor, especially if you have 3 or more cousins that you are comparing.
Third cousins are related through 2nd great-grandparents, of which we have 8. This relationship is a little more tricky to identify but still doable, particularly with 3 or more cousins to help narrow the search.
Focus on the most closely related DNA matches you have. If you have unknown relatives that share 100cM or more with you, there is a good chance you may be able to identify your common ancestor, even for beginners to genetic genealogy. For matches that are 30 cM or less, it is very difficult without a lot of corroborating matches.
So focus on finding your 2nd-4th cousins, and you might be able to have a bigger family reunion.