On 2008.04.03, Robin Altwarg <robin@...> asked:

[...]If my
cousin and I had our mtDNA tested would we be able to confirm or
disconfirm whether we share a great-grandmother? If so, do we have
to take a particular type of test?

If your mother's mother's mother and your second cousin's mother's
mother's mother were the same woman, then your mtDNA and your second
cousin's mtDNA would be identical (barring a tiny, tiny, tiny chance
of a mutation).

If you and your second cousin both have the basic mtDNA test
(hypervariable region 1, abbreviated as HVR1) and the results are
different, then you have different great-grandmother's on the direct
maternal line.

If you and your second cousin have the mtDNA test and the results
are the same for HVR1, there is a good chance that you had the same
g-grandmother. However, there are a few possible exceptions. If
your g-grandfather's second wife was a sister of his first wife or a
cousin of his first wife (who shared the same maternal grandmother
as his first wife), then both wives would have had the same mtDNA.

Another possibility is that your mtDNA HVR1 result and your second
cousin's is a very common one; then you and your second cousin could
share the same results by chance. In that case, you and your second
cousin could order further mtDNA testing (HVR2 and if necessary, the
full genomic sequence, FGS). If the results no longer match, then
you have two different great-grandmothers. If you match on HVR1 &
HVR2, then the chance is greater that you both have the same
g-grandmother. If you match on the FGS, then your g-grandmothers
are most likely the same person, or sisters or cousins on their
direct maternal line.

My recommendation would be for you and your second cousin to order
the mtDNA basic HVR1 test and, based on the results, decide if you
want to test further.

Hope this helps
Judy Simon