File talk:Table of Consanguinity showing degrees of relationship.png
From a genetic point of view, this table isn't even close to being correct. For example the "degree of relationship" between a person and his or her sibling (approx. 50% from a genetic standpoint) is much closer than between a person and his or her grandparents (approx. 25% from a genetic standpoint), yet the table outlines them to be of the same degree.
A person is equally much related to his or her parents, as he or she is to his or her siblings (1/2, or 50%, shared genetic material). Then everything follows nicely from this fact. Thus, 1/2 times 1/2, that is 25%, to grandparents, grandchildren, nieces, nephews, uncles, aunts. Then 1/2 times 1/2 times 1/2, that is 12.5%, to cousins, children of half siblings, grandchildren of siblings, parents of grandparents, and children of grandchildren.
Source of error described above
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Whoever made this table forgot that because full siblings share both parents, the amount of shared ancestry (in modern scientific terms, think: average amount of distinctive genetic material shared) needs to be multiplied by two, thereby decreasing the degree of consanguinity by one (1/[2^(n + 1)] * 2 = 1/[2^n]). That's the source of the numerous mistakes: Where full siblings are involved, the first generation descended from the common ancestor (i.e., the highest "dangler" in each column except that headed by the self) is of the same degree of consanguinity as is the common ancestor because there are really two common ancestors.
If sharing of only one parent were usual and if English had evolved to express relationships by reference to a norm of only one common ancestor rather than two, this table would be correct, and what English calls a "half-sibling" would be a "sibling," making what English calls a "[full] sibling" instead a "double sibling" (cf. "double cousin").
