There are tons of relationship charts floating around the web and they each serve a useful purpose…

Relationship Charts on the Internet #OnGenealogy

but ISOGG, the International Society of Genetic Genealogy, has a simple formula for understanding and naming your fundamental relationship to another person.

This isn’t intended for direct line relationships, Parent-Child, Grandparent-Grandchild or even some collateral line relationships such as Aunt-Niece, Uncle-Great Nephew, etc. It works, but those relationships are already nicely named.* This formula is for all types of collateral line cousins; DNA matches you don’t quite know how to name.

You may need to draw a family tree to diagram your relationship with another person. With that in hand, the ISOGG Wiki formula will tell you your degree of cousinship.

A family tree diagram showing my relationship to a recent DNA match. We’re both related to my great-great-grandfather. He’s my great-grandmother’s sister’s son. 

What do I call my relationship to this DNA Match? #OnGenealogy

ISOGG’s formula for How to Determine Your Degree of Cousinship with a DNA Match

      • Find your & your DNA match’s Common Ancestor, In the image above, our common ancestor is John Roberts

      • Next, determine your degree of cousinship. Of the two people you’re trying to determine the relationship between, select the person who is the closest to a Common Ancestor. This is the person who determines the degree of cousinship.  In the image above, Kenneth Robert Anderson is closer to our common ancestor than I am, so he determines our relationship.

      • Count the G’s. “Each “great” or “grand” has a numerical value of 1.” This number becomes the ordinal in the name of your relationship, “first cousins”, “second cousins”, “third cousins”, etc

        • If it’s a Grandparent (there’s one (1) in Grandparent), you and your match are first cousins In the image above, Kenneth calls our ancestor Grandpa, so 1G, so we’re 1st cousins

        • if it’s a Great Grandparent (there are two (2) G‘s in Great Grandparent), you and your match are second cousins

        • if it’s a Great Great Grandparent (there are three (3) G‘s in Great Great Grandparent), you are third cousins, etc

    • Count the number of generations that separate you from your DNA Match. If you’re from a different generation than your match, it’s called being “removed”, and the difference in generations determines how many steps removed you are from each other. Express any difference in generations between you and this DNA Match as an ordinal of the number of generations “removed” you are from this person (from your DNA Match, not from your common ancestor)

      • if you’re both the same generation, you’re zero generations removed, you’re just cousins (first cousins, second cousins, third cousins from your determination above)

      • In the image above, Kenneth and I are from different generations. I call our common ancestor Great-Great-Grandpa (3G) and he calls this person Grandpa (1G), my ordinal is 3G and his is 1G. I take the difference, 3G – 1G = 2G to find there’s a 2 generation difference between us. Our relationship is 1st cousins twice removed or 1st cousins 2 times removed.

#GenealogyTips Are We Cousins? Cousin Calculator #ISOGG #DNA #OnGenealogy

It’s counterintuitive to call someone two generations older than me a first cousin, that’s why I love this formula. It helps me reason through the relationship so I understand how and why our relationship is named the way it is.

*Technically, this formula is for all relationships, but no one calls their parent a “minus one cousin once removed”; a sibling a “zeroth cousin zero times removed”, or a niece or nephew a “zeroth cousin once removed”.

Best in your family history and genealogy!

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