Family relationship
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The family relationship between two individuals a and b, where Ga and Gb respectively are the number of generations between each individual and their nearest common ancestor, can be calculated by the following:
- x = min (Ga,Gb)
- y = |Ga-Gb|
- If x=0 and y=0 then they are the same person.
- If x=0 and y=1 then they are parent and child.
- If x=0 and y=2 then they are grandparent and grandchild.
- If x=0 and y>2 then they are great ... great-grandparent and great ... great-grandchild, with y−2 greats.
- If x=1 and y=0 then they are siblings (brothers or sisters).
- If x=1 and y=1 then they are uncle/aunt and nephew/niece.
- If x=1 and y>1 then they are great ... great uncle/aunt and great ... great nephew/niece, with y−1 greats.
- If x>1 and y=0 then they are x−1th cousins.
- If x>1 and y>0 then they are x−1th cousins y times removed.
So two people sharing a pair of grandparents have x=2 and y=0 and are described as being first cousins.
If x>0 and they only share one nearest common ancestor rather than two, then the word "half" is sometimes added at the beginning of the relationship.