Do you have unknown DNA matches in the 800 centimorgan range? This is a follow-up to my previous post called Four Hundred Centimorgans. I've worked with many people who have big DNA matches that have never been investigated. This isn't something that should be ignored. When you get into the 800cM range, things really become clear about the relationships. Make sure you know who those big DNA matches are. Once again, keep in mind, I'm using nice round numbers here to keep it simple. Numbers of all different sizes can be important, but 800+ will surely result in a close relationship. Numbers in the 600s or 700s are also pretty darn high. I strongly recommend you find out who those people are if you don't know them. Also, additional things are possible other than what I'm about to mention, but these are the most likely scenarios. Note: I'm using centimorgans as the unit of measurement. If you're using 23 and Me, it may only show percentages. 800 centimorgans is a little less than 12 percent.
Do you know your close family? By that I mean; parents, siblings, aunts, uncles, first cousins, nieces and nephews. If you do, and you see a match in the 800cM range, you should know that person. If you don't know them, find out who they are. It will be a big deal. In fact, it could be a VERY big deal. Maybe even a life altering situation for someone in your family. Sounds a little scary, right? I'll explain... In my previous article, I talked about how 400 centimorgan matches often result in a relationship closer than second cousins. Now we're talking about very close family. This will likely be a person who has two grandparents in common with you. If the person is in the same generation as you, they should be your first cousin. That's the easy answer, and its usually what people think of when they see numbers in this range. Remember, the relationship can only be first cousin if the person is in the same generation as you. What if they are in an older or younger generation? That's where things start to get really interesting...
IF YOU KNOW YOUR FAMILY, BUT DO NOT KNOW A MATCH THIS HIGH, DO NOT IGNORE IT!
In a previous post I talked about half cousins and how that relationship is often overlooked and misunderstood. The same is true here as well. This large match often results in a half aunt-uncle or a half niece-nephew relationship. What does that mean? A half aunt or uncle is a half sibling of your parent. Therefore, they are one generation older than you. A half niece or nephew is a child of your half sibling. Therefore, they are one generation younger than you. As you can see, this is a big deal. If you don't know the person with the 800+ match, it means you might discover your parent has a half sibling or, YOU have a half sibling. I've helped people with this type of match who've discovered their father wasn't who they thought he was, which explained why they had an unknown half sibling.
Again, I want to stress that DNA numbers can vary and I'm using a nice round number for this example. This is a likely situation with a number in that range though. I've seen it happen many, many times. People discover half siblings all the time and it's often because of matches like this. I'll soon be posting more about DNA numbers and ranges. Just to give you an idea about how the numbers can vary, I've seen this relationship occur with numbers anywhere between 600 and 1200 centimorgans. when you get to the higher or lower end of that range, other relationships become more likely, so don't jump to any conclusions. 800 is a pretty average number to see for that relationship though. As with any DNA match, always investigate thoroughly, and feel free to ask me for help if needed.
I'll share a brief story involving a match like this. I see this type of thing happen more often than you can imagine. Someone was looking for a family member who had been missing for a long time, a half-sister. I happened to answer a posting in a newsgroup about this from many years ago. They did Ancestry DNA and had a match to someone with 800cM on their list. They had no idea who it was and never bothered to find out. I took a quick look at this person's DNA list and realized it was a half niece. The mother of the half niece was the missing half-sister. After a little more research we were able to find the half-sister and contact her. The answer to this person's mystery was right in front of them the whole time. So, once again, never ignore these big matches.
Next we look at the importance of thorough investigation.