Wednesday, January 9, 2013

Ancient Mitochondrial DNA

Another interesting match in the database at 23andMe!

A. H. was adopted and has no genealogy to share.  She matches my father's cousin on a small segment on chromosome 22.

Using the new Ancestry Composition feature available at 23andMe, we can see that her ancestry is roughly three quarters European and one quarter Asian.  This reflects at least five generations on all of her ancestral lines.

23andMe
Ancestry Composition of distant cousin A. H.

Previously written in the WEIRD post, most people in the database are of European ancestry.  So the first observation we can make about A. H. is that her genetic cousins at 23andMe will be of European ancestry and not Asian because that is the population in the sample.  Very few people of Asian heritage are available for her comparison.

Next we get to the maternal haplogroup of A. H.  Maternal haplogroups are named with letters and numbers and reflect ancient migratory paths tens of thousands of years ago when people lived, reproduced, and died within the same few miles of their parents and their parents and so forth.  The time before ships, trains, planes, and cars could quickly transport people all over the world.  This is great for anthropologists.  This is not really relevant for genealogists.  Written records of genealogical value are scarce as we travel back in time; some places simply have no surviving records.  Learning your maternal haplogroup, or where one of your thousands of ancestral lines lived 30,000 years ago, is not going to help you uncover the name of Mary Thompson's husband whom she married in between the 1870 and 1880 census.

Current DNA analysis of mitochondrial haplogroups shows that people originated in Africa
and traveled (very gradually) across the globe.  After thousands of years,
the DNA of mitochondria gradually mutated within sequestered groups,
giving rise to different haplogroups.


The Science:  Maternal haplogroups are determined by mitochondria.  They are the "powerhouses" of cells, if you remember back to your biology class.  Mitochondria contain their own unique DNA separate from the DNA contained in the cell's nucleus.  DNA in the nucleus is reconfigured, or recombines, when sex cells are created in preparation for reproduction, with only half the cell's original DNA getting passed on to offspring.  In contrast, the DNA in mitochondria does not change except for tiny mutations that occur every few thousand years.  You have your mother's mitochondrial DNA and not your father's because the mitochondria in sperm are destroyed before or at conception.

Drawing of mitochondria.


A. H.'s maternal haplogroup is L1b1a6.  This is interesting because this is a Western/Northern African haplotype, yet A. H. has no detectable African heritage in her.  If A. H. were able to trace her direct maternal line, she would eventually end up in this area of the world.  The problem is that this may take hundreds of generations, which is not possible with the written records available to us.  No records about A. H. are available to her because she was adopted.  23andMe has provided her with her first glimpse of her origins.


23andMe

Knowing your maternal or paternal haplogroup is kind of like getting the answer, but not the equation.  We are skipping over thousands of years of genealogy not recoverable to us and seeing where one of our lines was living.