Lots of people struggle with figuring out exactly how two people are related.
Most genealogy programs include a relationship feature, but what if you are working with a new genetic cousin whose line isn’t yet in your genealogy software? Hopefully, that happens often!
There are also nice reference charts available, like this one provided by Legacy Tree Genealogists.
However, rather than trying to figure out who fits where, it’s easier and quicker for me to quickly sketch this out by hand on a scrap piece of paper. I can do this while looking at someone’s tree or an e-mail much more easily than I can deal with charts or software programs.
Rather than make you look at my chicken scratches, I’ve typed this into a spreadsheet with some instructions to make your life easier.
Common Couple Ancestor
This first example shows a common couple ancestor – as opposed to calculating a relationship to someone where your common ancestor’s children were half siblings because the ancestor had children by two spouses.
Down one side, list your direct line from that ancestor couple to you.
On the other side, list your matches direct line from that ancestral couple to them.
The first generation, shown under relationship, will be siblings.
The next generation will be first cousins
The next generation will be second cousins, and so forth.
You can see that Ronald and Louise are one generation offset from each other. That’s called “once removed,” so Ronald and Louise are third cousins once removed, or 3C1R.
If Ronald’s child had tested, instead of Ronald, Ronald’s child and Louise would be third cousins twice removed, because they would be two generations offset, or 3C2R.
See how easy this is!
Half Sibling Relationships
In the circumstance where Ronald and Louise didn’t share an entire ancestral couple, meaning their common ancestor had a different spouse, the relationship looks like this:
The only difference in the relationship chart is that Jane and Joe are half siblings, not full siblings, and each generation thereafter is also “half.”
The relationship between Louise and Ronald is half third cousins once removed.
It’s easy to figure relationships using this quick methodology!
Update: I can tell from the comments that the next question is how much DNA to these various relationships share, on average. The chart below is from the article Concepts – Relationship Predictions, where you can read more about this topic and the chart.
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