Relationships are not always about biology.
It’s important in genealogical research to be aware of the biological relationships between two individuals as those relationships are the ones that may leave genetic evidence behind. That genetic evidence is strongest the closer the relationship is. The further the connection, the lower the chance that an autosomal DNA test finds genetic evidence of the connection.
Biological relationships are not the only ones that matter in genealogical research.
There are relationships established through marriage. There are relationships established through a shared heritage, growing up in geographic proximity, being members of a shared religious tradition, etc. Anyone who has that relationship to your ancestor or relative may interact with him in ways that leave additional records behind and may help you to learn more details about your ancestor’s life.
And there can easily be multiple relationships between two individuals. They may be biologically related to each other in more than one way (double cousins, for instance). They may be related to each other by marriage and by biology (where a husband and wife are third cousins).
Relationships cause people to interact with each other and that interaction can result in items of genealogical interest: either generating more humans, more records, or both.
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