SUPER SIMPLE CENTIMORGANS

Blog Image May 28, 2024

I've mentioned several times that people are not taking their large unknown DNA matches seriously.  I'm seeing it so often, it's actually a little frustrating.  I really can't believe it.  If a 1000cM DNA match showed up on my list and I didn't know them, I don't think I could sleep until I found out who it was.  Maybe that's just me.  Maybe some people just don't care, but I think the main reason people don't take this seriously is because they just don't understand that they really are closely related to all these people.  My most recent case involved someone who didn't know who his father was.  He had several matches over 500cM and several more over 1000cM.  Somehow, no one even questioned him about who he was or why he's on their match list.  So, I thought this post might help people understand that this is a big deal.

In previous posts I've shown various charts with lots of centimorgan and relationship data.  See my previous post called DNA Numbers Lesson.  That might be a little intimidating for some people, so I decided to make a super simple table showing some really basic data.  This is as easy as it gets.  Even if you know nothing about DNA, this should at least make you say, "Hey, I need to look into this."  This is a very scaled down version of a centimorgan chart to only show you the big numbers and important relationships.

cMYounger
Generation
Same
Generation
Older
Generation
1400-
2000
Niece or
Nephew
Half SiblingAunt or
Uncle
800-
1000
Half Niece or
Half Nephew
First CousinHalf Aunt or
Half Uncle
400-
500
First Cousin Once RemovedHalf First CousinFirst Cousin Once Removed
200-
300
Half First Cousin Once RemovedSecond CousinHalf First Cousin Once Removed

Do you know your family?  If so, check your match list.  See any matches with large numbers (over 400cM) who you don't know?  The chart above is very simplified and will show you possible, or should I say "probable" relationships to your DNA match.  The numbers and the generation are what's important.  Since its most common for DNA matches to be in the same generation, or one generation older or younger, I've only included those three generations in the chart.  You may not know the age or generation of you DNA match, but no matter what, they will still be closely related with these numbers.

Let's say you see an unknown DNA match with 900cM on your list.  If he has a tree made and his parent's and grandparent's ages look similar to yours, you can probably assume he's in the same generation as you.  He should be a first cousin.  If you know all your first cousins and don't know this person, it's probably a big deal, right?  It would mean one of your parent's siblings had a child you don't know about.

Notice I didn't include centimorgan numbers covering every possibility.  Like 600cM isn't on the chart.  That's because that's an "in-between" number and can fall between different relationships.  It's still going to be important, but I'm really trying to keep it simple here.  I didn't want to include a big chart with everything possible.  The only thing that may not be important here is the last row of the chart.  You might be getting into second cousin range there and some people just don't care about second cousins.  However, if you know who your great-grandparents are, and you see a match in this range, and none of their great-grandparents have any common names in their tree, it could end up being a big deal for someone. You should see common names if you have matches in this range.  Just to be sure you understand each relationship mentioned, let's go over all the ones we see in the chart.

Aunt, Uncle, Niece, Nephew and Half Sibling - These are obvious.  No need to explain these.

Half Niece/Half Nephew - This person will be a child of your half sibling.  If you're thinking, "Wait, I don't have a half sibling!"  Well, that's why this is important.  You might want to check into this.

First Cousin - This person will have two grandparents in common with you.  Or, another way to think about this is, they will be a child of your parent's full sibling.

Half Aunt/Half Uncle - This is a half sibling of your parent.  If you're not aware that your parent has a half sibling, again, you'll want to check into this.

First Cousin Once Removed - There are two ways to think about this relationship.  Someone who is your parent's first cousin, or a child of YOUR first cousin will be your first cousin once removed.

Half First Cousin - Someone who has ONE grandparent in common with you will be your half first cousin.  See my previous post called Four-Hundred Centimorgans.  I talk about why this relationship often results in some interesting discoveries.

Half First Cousin Once Removed - Similar to the first cousin once removed relationship shown above.  This will be a child of your half first cousin, or a half first cousin of your parent.  Even though centimorgan numbers in this range are starting to get low, unexpected relationships could result.  Investigate thoroughly and see if you can explain DNA matches you find in this range.

Second Cousin - Someone with two great-grandparents in common with you will be your second cousin.  Another way to think about that is, their grandparent will be a sibling of your grandparent.

This is a super simplified example of some possible relationships.  The goal here is to get people to look at all their high DNA matches and figure out who they are.  It could result in an important discovery for someone.  My chart is based on common numbers and relationships I often see.  DNA results can sometimes be unusual and different relationships could result, but these are quite common.  Good luck.

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