Double Cousins

I recently came across a report of a Dooley cousin in St. Louis - Alex Dooley, Hamburger Man in St. Louis - (though I haven't yet contacted this family and they may not be aware of our connection). My Legacy Family Tree software tells me that Alex's children are my fourth cousins, through two different paths, i.e., double fourth cousins.  I set out to find out what that means genetically and if there is some sort of metric to allow me to compare a "double fourth cousin" to the more common single fourth cousin.  There is a Coefficient of Relationship, R, related to degrees of relationship, but the math might be too much, so first I'll skip to the results, then try a brief basic explanation, then point to some resources for more information, if you're interested.


Single relationships

Siblings have about half of their genes in common, the degree of relationship is 1 or first, and the corresponding coefficient of relationship, R, is 1/2.  Advancing one generation: first cousins have in common about 1/8 of their genes, the degree of relationship is 3, and the corresponding R is 1/8. Each consecutive generation shares just 1/4 as many genes as the previous generation, the degree increases by 2, and the corresponding R is only 1/4 as large.  The following table shows these values through fourth cousins.

Relationship	Degree	R	% genes in common
Self or identical twins	0	1	100
Siblings	1	1/2	50
1st cousins	3	1/8	12.5
2nd cousins	5	1/32	3.1
3rd cousins	7	1/128	0.8
Double 4th cousins	8	1/256	0.4
4th cousins	9	1/512	0.2

Our double relationship

So, where does the "double" come in? Back in 1863, William Dooley married Elizabeth Martin in St. Louis.  In 1887, William's niece, Anastasia LaBrune, married Elizabeth's nephew, James Hogan. This created a double relationship between the Dooleys and the Hogans. William and Elizabeth's son, Thomas, was a first cousin to both Anastasia LaBrune on his father's side and James Hogan on his mother's side.  As an aside, since Thomas was an only child AND the Dooley's were Anastasia's only family in St. Louis AND Thomas and Anastasia were only four years apart in age AND James Hogan was also family AND the Hogan kids and Thomas' kids were all close in age, the Hogans and Dooleys were probably very close, akin to siblings, at least in their teen and adult lives.  In the next generation, Thomas' kids were second cousins to the Hogan kids, once through Anastasia and the Dooleys and again through James and the Martins.  This made them double second cousins. The next generation were then double third cousins, and so forth. How does that change the values in the table? Basically this means that instead of having one set of ancestors in common, they have two, both the same number of generations back, so the descendants of Thomas Dooley and of James and Anastasia LaBrune Hogan all have twice as many genes in common. The degree of relationship for double fourth cousins in 8, R is 1/256, and they have about 0.4% of their genes alike. According to one of the sources listed below, this is about 117 genes of the approximately 30,000 in the human genome.

More about quantifying relationships

If you'd like to know more, perhaps about how to include half siblings, or how to trace out any relationships, here are some explanations on the WWW:

Genetic and Quantitative Aspects of Genealogy
A thorough explanation of the Coefficient of Relationship (R) and related subjects.

Quantitative Consanguinity
A less through explanation with more applications to genealogy, but they only show degrees of relationship through 7, whereas a fourth cousin is degree 9.

Degrees of Relation and Number of Genes Shared
Not thorough, but relates R to the number of genes shared for various relationships.