L. David Roper (firstname.lastname@example.org)
There are several reasons why the Y-chromosome markers for two males
with the same surname are not be the same:
- The surname may have been created independently at different places
in the ancestry of the two males.
- Y-chromosome mutations occur at a slow rate. If the common ancestor
of the two males with the same surname goes back to the origin of the surname,
say 500 years ago or about 20 generations ago, there could be several relative
mutations for 25 Y-chromosome markers between the two males. If the surname is
very old, say it originated 1000 years ago or about 40 generations ago, relative mutations between two males could be twice as much or more.
- One of the males may have a male-line ancestor that was adopted and
has a father with a different surname. This information may not be in the
- One of the males may have a male-line ancestor that was sired by a
male with a different surname, but was given the surname in question with or
without the knowledge of his supposed father. According to the book Sperm Wars by Robin Baker: "On average, about 10
percent of children are not sired by their supposed father."
The later two reasons I call "surname breaks."