Human Genealogy
The coefficient of relationship is sometimes used to express degrees of kinship in numerical terms in human genealogy.
In human genealogy, the value of the coefficient of relationship is usually calculated based on the knowledge of a full family tree extending to a comparatively small number of generations, perhaps of the order of three or four. As explained above, the value for the coefficient of relationship so calculated is thus a lower bound, with an actual value that may be up to a few percent higher. The value is accurate to within 1% if the full family tree of both individuals is known to a depth of seven generations.
| r | relationship | degree of relationship |
|---|---|---|
| 100% | identical twins; clones | 0 |
| 50% (2−1) | parent-offspring | 1 |
| 50% (2−2+2−2) | full siblings | 2 |
| 37.5% (2−2+2⋅2−4) | 3/4 siblings or sibling cousins | 2 |
| 25% (2−2) | grandparent-grandchild | 2 |
| 25% (2−2) | half siblings | 2 |
| 25% (2⋅2−3) | aunt/uncle-nephew/niece | 3 |
| 25% (2−3+2−3) | double first cousins | 4 |
| 12.5% (2−3) | great grandparent-great grandchild | 3 |
| 12.5% (2⋅2−4) | first cousins | 4 |
| 12.5% (8⋅2−6) | quadruple second cousins | 6 |
| 9.38% (6⋅2−6) | triple second cousins | 6 |
| 6.25% (2−4) | half-first cousins | 4 |
| 6.25% (2⋅2−5) | first cousins once removed | 5 |
| 6.25% (4⋅2−6) | double second cousins | 6 |
| 3.13% (2−6+2−6) | second cousins | 6 |
| 0.78% (2⋅2−8) | third cousins | 8 |
| 0.20% (2⋅2−10) | fourth cousins | 10 |
From the above table, it can be seen that most legislation regarding incestuous unions concern relations of r = 25% or higher, while most permit unions of individuals with r = 12.5% or lower. An exception are certain US states where cousin marriage is prohibited. Also, most legislations make no provision for the rare case of marriage between double first cousins. It should also be noted that incest laws also include prohibitions of unions between unrelated individuals if there is a close legal relationship such as adoption.
Read more about this topic: Coefficient Of Relationship
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