Mitochondrial DNA (mtDNA) as we mentioned earlier, is DNA that is only inherited from the mother. It is not the X chromosome, it is actually in the mitochondria of a cell. mtDNA is non-nuclear DNA, so it is not part of your chromosomes or “genes” and is passed “unchanged” (except for SNP mutations that occur) from mother to all children, but can only be passed on by a mother, so a mtDNA shows the distant female-line descent. Genealogy mtDNA tests are for SNPs. mtDNA SNPs are not very numerous, so the branches are quite infrequent. Perfect matches may still not have had a common female-line ancestor for 500 or 1000 years. It is most useful on very old specimens, as samples have remained stable to test for many thousands of years.
At first mtDNA was compared by mutations against the revised Cambridge Reference Sequence (rCRS), which is the Haplogroup H2a2a, but now uses Letter and number Haplogroups just like Y-DNA.
Some of the Y-DNA groups we mentioned earlier have equivalents in mtDNA– Y-DNA Haplogroup I is probably the same population source as mtDNA U Haplogroup, and Y-DNA R1 is probably the same population source as mtDNA H, and specifically H2a1 mtDNA may be same population as R1a Y-DNA. 50% of Europe today is mtDNA H.
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