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
mtDNA Part V - Detecting Mutations in the mtDNA
mtDNA Part VI - mtDNA Ancestral Markers
mtDNA Part VII - The Cambridge Reference Sequence  <<– you are here
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 discuss the Cambridge Reference Sequence (aka CRS).  The CRS is a fundamental part of mtDNA data analysis.  A basic understanding of the CRS and how it is used in determining mutations will allow you to understand the role that mutations play in tracing your ancestry. 

What is the CRS?

The CRS is the first human mtDNA that was ever fully sequenced and published.  The work was performed by scientists at Cambridge University, and this groundbreaking study was officially published in 1981.  Click here to view a copy of the original publication.

This publication represents the first time that the mtDNA was sequenced.  The donor whose DNA was used for this ground-breaking project was of European descent and belonged to European Haplogroup H. 

Since this was the first mtDNA sequence ever published, this sequence was thereafter refered to as a “reference sequence” upon which all further mtDNA sequences from labs around the world was compared to.  This original sequence eventually came to be known as the “Cambridge Reference Sequence” and all mtDNA which is sequenced, even today, is compared to the CRS.

Mutations are determined based on comparison with CRS

When we state that we have mutations in our mtDNA, we are actually showing the regions of our DNA which differ from the CRS.  Let’s take a look at an actual mutation report:

In this report, the HVR1 region was tested, and 6 mutations were detected, indicating that this individual’s HVR1 region differs from the CRS at 6 different locations.  Let’s take a look at the first mutation in the list:  16126 T>c.  This means that the individual’s mtDNA is different from CRS at location 16126.  It shows that CRS has a “T” at this location, but the person tested has a “C”. 

Let’s look at the same results based on the sequencing report:

All of the letters in “black” are the same as CRS.  All of the nucleotides in “Red” are different from CRS and are considered “mutations”. 

We are all compared to the CRS, not the earliest human mtDNA!

The key point to remember is that when the results of mtDNA testing are used for genealogical purposes, the results are compared to the CRS and mutations are reported as “differences” between the results and the CRS. 

This however, can lead to confusion for beginner genetic genealogists because instinctively, people usually think that when scientists look for mutations, they should be comparing the our mtDNA to that of the earliest human DNA to see how our DNA has changed over time.  However, that is not how the research community has decided to approach the mtDNA.  The consensus within the scientific community is that mtDNA is always compared to CRS.  Since this is the case, it is important for you to become familiar with how this “reverse” method is used to analyze our mutations and determine haplogroups. 

The role of CRS in haplogroup determination:

Let’s take a look at the human mtDNA haplogroup tree.  This is a phylogenetic tree which shows how all people living today descended from a common ancestor (mitochondrial eve) who lived in Africa over 150,000 years ago.  Every person living today can trace his/her ancestry to a branch of this tree, called a “haplogroup”.  The European individual whose mtDNA sequence is famously called the CRS is located at a distant branch of the tree as shown in the diagram below:

Where is the CRS located on the mtDNA haploplogroup tree?

Now, let’s take a look at how your mtDNA haplogroup is determined. 

To determine your mtDNA haplogroup, always start with the CRS and move away. 

Example #1:  If you HAVE mutations at locations 263 and 7028, and DO NOT have mutations at locations 14766 or 16067 or 16298, then you belong to Haplogroup HV:

Example #2:  If you HAVE mutations 263, 7028, 14766, 73, 11251, 16126, and 16069, and DO NOT have a mutation at 16294, then you belong to Haplogroup J. 

Example #3:  If you HAVE mutations 263, 7028, 14766, 73, 11719, 12705, 16223, 10873, 2352, and 150, then you belong to Haplogroup L3e:

Summary of procedure for determining mtDNA Haplogroups:  To determine your haplogroup, always start from the CRS and move backwards in the tree to see which mutations you have and which ones you do not have.  Your haplogroup is determined by the difference between your markers versus CRS. 

What if I don’t have any mutations?  If you do not have any mutations, that means that your mtDNA sequence (at least the part that was tested) is exactly the same as CRS.  CRS belongs to a branch of Haplogroup H, so if you belong to Haplogroup H, chances are that you will not have too many mutations in comparison with CRS.

This concludes the basic overview of the CRS.  In the Part VIII, we will start going over the different mtDNA test types available, and what each test type will tell you.