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|
|Double 4th cousins||8||1/256||0.4|
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.
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.