The Road to Ethel

as travelled by Brians, Pences, Bennetts, Clemmonses, Swifts, et al

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Kinship and Degrees of Relatedness


When I got started on this project, I had a lot of questions about "degrees of relatedness" when it comes to kinship.  By degree of relatedness I am referring to DNA similarity. For instance, how much DNA do I share with a first cousin? Or with a great-great-great-great grandparent? What is the difference in kinship between a first cousin, second cousin, and a second cousin once-removed, and so on, and how does that distinction factor into shared DNA? I quickly learned that last names don't seem to play much of a role in answering any of these questions. So let's start off with each of the basic kinship definitions and then move from that into the degree to which each type of relationship is genetically related.


Direct Ancestors


Direct ancestors are parents, grandparents, great-grandparents, second great-grandparents, and so on. With regards to degrees of relatedness, each successive generation of direct ancestors has half the shared DNA of the one before it. I share 50% of my DNA with each of my parents. We'll call that a .5 degree of relatedness. I share 25% of my DNA with each of my grandparents (.25 degree of relatedness.) I share 12.5% it’s my great grandparents, and so on. As a point of reference, I have a .039 degree of relatedness with my direct ancestor, William Brian of County Cork, which is the same degree I share with my 4th cousins or 3d cousins twice-removed. Yeah. I was surprised, too.


Siblings


Degree of relatedness between siblings is the same as for parents and children: .5.


Aunts and Uncles


Aunts and uncles follow the same basic rule as for direct descendants. Each successive generation of aunts and uncles has half the shared DNA of the one before. Aunts and uncles start off with a .25 degree of relatedness. Great-aunts and -uncles are at .125, and so on.


Cousins


Cousinship requires a bit more explanation. Here are the basic rules: 1) First cousins share a common set of grandparents. In other words, they are the children of your aunts and uncles. 2) Second cousins share a common set of great-grandparents, but not the same grandparents. 3) Third cousins share a common set of great-great-grandparents, fourth cousins have the same great-great-great-grandparents, and so on. 4) The word "removed" indicates that the two related people are from different generations.  For example: **1x removed means a difference of one generation. So, your mother's first cousin is your first cousin, once removed. **2x removed means a difference of two generations. So your grandmother's first cousins are your first cousins, twice removed, and so on.


The following chart does a pretty good job of explaining all this.




Kinship and Degrees of Relatedness




Owner/SourceAlicia Clemmons

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