) between two individuals B and C is obtained by a summation of coefficients calculated for every line by which they are connected to their common ancestors.
The coefficient of relationship is sometimes used to express degrees of kinship in numeric terms in human genealogy.
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.
[c] A first-degree relative (FDR) is a person's parent (father or mother), sibling (brother or sister) or child (son or daughter).
It includes uncles, aunts, nephews, nieces, grandparents, grandchildren, half-siblings and double-first cousins.
[4][5][6] Third-degree relatives are a segment of the extended family and includes first cousins, great-grandparents and great-grandchildren.
[8] The category includes great-grandparents, great-grandchildren, granduncles, grandaunts, grandnephews, grandnieces, first cousins,[9] half-uncles, half-aunts, half-nieces and half-nephews.
Most incest laws concern the relationships where r = 25% or higher, although many ignore the rare case of double first cousins.
The kinship coefficient is a simple measure of relatedness, defined as the probability that a pair of randomly sampled homologous alleles are identical by descent.