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Which relatives are considered consanguineous because they share common ancestors?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

Consanguinity refers to kinship defined by the sharing of common ancestors. While the term historically implied shared "blood," it is understood in modern contexts as a genetic relationship where individuals inherit genes from the same ancestral source.1

Consanguineous relatives are categorized by their degree of genetic relationship:

First Degree: Includes parents and children, as well as siblings, who share a high proportion of their genetic material.2
Second Degree: Includes aunts, uncles, nieces, and nephews, who share approximately half the genetic commonality of first-degree relatives.2
Third Degree: Includes first cousins, who share one-eighth of their genes.2

These relationships are further distinguished as either lineal (direct ancestors and descendants, such as a great-grandparent and great-grandchild) or collateral (relatives who share a common ancestor but are not in a direct line of descent, such as cousins). Notably, consanguinity excludes individuals who do not share common ancestors, such as those related solely by marriage (affinal kin) or adoption.31