DNA Lesson Series: The mtDNA and its role in Ancestry
mtDNA Part I - mtDNA 101
mtDNA Part II - Facts about mtDNA
mtDNA Part III - mtDNA Structure
mtDNA Part IV - Ancestral Markers  <<– you are here
mtDNA Part V - Detecting Mutations in the mtDNA
mtDNA Part VI - mtDNA Ancestral Markers
mtDNA Part VII - The Cambridge Reference Sequence
mtDNA Part VIII - mtDNA Test Types
mtDNA Part IX - mtDNA Haplogroup Determination
mtDNA Part X - mtDNA Subclades
mtDNA Part XI - mtDNA Haplogroup H
mtDNA Part XII - Subclades of mtDNA Haplogroup H
mtDNA Part XIII - Distribution of Subclades of H
mtDNA Part XIV - Descendents of Maria-Theresa
mtDNA Part XV - Luke the Evangelist
mtDNA Part XVI - Empress Feodorovna
mtDNA Part XVII - James “Earthquake McGoon” McGovern

In this blog, we will talk about ancestral markers, what they are and where they are found in the mtDNA.

What is an ancestral marker? 

mtDNA is a circular chain consisting of 16,569 pairs of nucleotides.  Let’s unwind the DNA double helix and take a closer look at its genetic code. 

DNA consists of two chains of nucleotides, designated A, C, T, and G.  “A” is always linked to “T”, and “C” is always linked to “G” on the opposite chain.  In this diagram, we will take a closer look at a short segment of mtDNA, namely locations 1 to 45.  The unique combination of nucleotides in the chain is called a “genetic code” and holds genetic information. 

Ancestral markers are “mutations”, little changes or “hiccups” that occur in the genetic code of the mtDNA.  There are many types of mutations, but the type of mutation most commonly found in mtDNA is called “SNP” (single nucleotide polymorphism).  A SNP mutation occurs when a single nucleotide is replaced with a different nucleotide.  For example, in this diagram, the “T” at location 40 is replaced by a “G”. 

This mutation is documented as follows:

• Location:  40
• Nucleotide Change:  T>G

When you test your mtDNA, your results report will document the mutations that you carry in your mtDNA.  Let’s take a look at a sample report:

In this report, “Location” refers to the locations on the mtDNA where a mutation has been detected.  “Mutation Type = Substitution” means that a nucleotide has been substituted by a different nucleotide.  “Nucleotide Change” indicates what was substituted.  Let’s take a look at the first mutation in the list.  Location “16126″ means that a mutation has been detected at location “16126″.  T>c means that a “T” has been replaced by a “c” at this location.

A second way to look at your results is to take a look at the actual sequence.  In this example, the sequence shows the results for the HVR1 region, from locations 16001 to 16520.  Remember, only one of the two chains in the pair is shown when reporting the sequence!

Fifty nucleotides are listed per line from left to right.  In this example, the first line shows the results for locations 16001 to 16050, the second line shows the results for 16051 to 16100, and so on.  Mutations in the sequence are highlighted in pink.  In this sequence, the nucleotide at location 16126 is highlighted in pink, indicating that a mutation is detected here, and the nucleotide has been replaced by a “c”. 

The unique set of mutations that you carry in your mtDNA holds information about your maternal ancestry.  In Part V, we will talk about the technology used to detect mutations in your mtDNA.