Why can Y chromosome tests help determine whether men with the same surname share an ancestor?

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Large illustrated genealogical tree with detailed branches and leaves, featuring family names and a central family crest, representing the Lippincott family lineage. — Genealogy Genealogical tree of the Richard and Abigail Lippincott family in America, constructed and published by Charles Lippincott, 1880.

Y chromosome testing is a powerful tool in genealogy because of the specific way the Y chromosome is inherited. In humans, the Y chromosome is passed exclusively from father to son, remaining largely unchanged across generations.

Because males who share a common paternal ancestor possess matching or nearly identical Y chromosomes, scientists can compare these genetic profiles to trace paternal lineages. This method allows genealogists to determine whether men who share the same surname are biologically related through a common male ancestor, even if their connection is distant and not documented in traditional written records.1