DNA Musings no. 24: Cousins: First, Second, Removed, and Half

By Betty Hagberg. When using DNA in genealogy, it is particularly important to understand the exact genetic relationship you have with your relatives.

Cousins

‘First cousins’ share a set of grandparents.  The children of ‘first cousins’ share a set of great grandparents and are ‘second cousins’.

First cousins share grandparents.

Second cousins share great grandparents.

Third cousins share great great grandparents.

One way to remember is to count the number of g’s. The phrase ‘great great grandparents’ contains three g’s and that means cousins sharing great great (2g) grandparents are third cousins.

Removed

What about ‘removed’ relationships?

Insert your own relatives in the following examples. “Removed” indicates that you and your relative are not of the same generation.  My ‘first cousin’ (1C) Tom has a daughter Susie.  Susie is my ‘first cousin once removed’ (1C1R). My grandparents are Susie’s great grandparents. We are ‘once removed’ because we are one generation apart.  My daughter is the same generation as Susie making them ‘second cousins’.

But wait, you are also a 1C1R with ‘first cousins’ of your parents.  My mother has a ‘first cousin’ Jeanette. Jeanette is my 1C1R.  My great grandparents are Jeanette’s grandparents.  What controls whether we are called first or second cousins since both great grandparents and grandparents are involved? The person closest to the common ancestor, in this case Jeanette and her grandparents, determines that we are designated first cousins and the fact we are one generation apart determines that we are ‘once removed’.

Half relationships

It can sometimes be confusing to determine if you have a ‘half’ or ‘full’ relationship with a relative. The key to determining this is finding the ‘common ancestor’ shared by two individuals. Work back in a family tree until you find an ancestor that both individuals are descended from. This is sometimes called the ‘most recent common ancestor’.

If the ‘common ancestor’ of two individuals is a couple: two parents, two grandparents, two great grandparents, then there is a full relationship.  If the ‘common ancestor’ is a single person: one parent, one grandparent, one great grandparent, then there is a half relationship. Half-siblings share only one parent.  Half-cousins share only one grandparent. Half-second cousins share only one great grandparent.

Why is this important?  Each genetic relationship is associated with a range of shared DNA. If the amount of DNA you share with a DNA match is consistent with their relationship as shown in your tree, you have additional evidence that your genealogical tree is correct.