Re: mtDNA and other DNA

["v.h.villarreal" <raices_regias@...>]

Josie,

That's a keen observation and a good question about the number of differences in our mtDNA in relation to the CRS reference.

Let's start by saying that the CRS (Cambridge Reference Sequence) is just an arbitrary reference selected by some team of geneticist that were among the first to study the genetic sequences of mitochondrial DNA.  They decided to use the sequence of one of their subjects as a comparison reference, maybe one of their own samples, who knows?.  The selected mtDNA belongs to haplogroup H, the most prevalent haplogroup in Europe.  Therefore many European persons who get tested have few differences if any with the CRS reference.

Next, what the commercial tests show us about our mtDNA is not the whole sequence of mtDNA but just one or two regions:  HVR1 and HVR2 (Hyper Variable Regions).  Supposedly these two regions don't have any medical implications so having a fewer more or a fewer less mutations is not very significant.  But that is not always the case in the remainder of the mtDNA molecule.  Some mutations could be good while other mutations could be bad in terms of health and disease.

To better understand what we're talking about let's use an analogy.  DNA sequences are a string of chemical bases limited to four possible values A, T, C and G (Adenine, Thymine, Cytosine and Guanine).  So think about a collar made up of coloured beads.  The beads come in just four colors: red, yellow, green and blue.    Now look at the following comparison between CRS and my own mtDNA in the 16310-16320 segment:

CRS Reference:   TACATAAAAC 
My mtDNA:        TACATAAAGC

The difference in my bead collar is obvious at the 9th position or 16319;  an A has been replaced by a G.  

Therefore, if I have two or more mutations in my mtDNA than you it probably means that somewhere along the way some of my ancestress acquired extra mutations while yours remained unchanged.  I said probably because there is such a thing as "back mutations" where a mutation reverts back to its original value or state but these are supposed to be even rarer than regular mutations.  We don't know yet exactly all the events that can cause these mutations to ocurr but we can sure use them to trace the branches of our big motochondrial tree.

Saludos Cordiales,
Victor


--- In ranchos@yahoogroups.com, "Josie T. Trevino" <Josiett3@s...> wrote:
>
> Victor......thanks for the explanation on DNA, and you do not sound
> patronizing.  I appreciate the detail that you provide regarding DNA.  I
> have been so involved in searching for my ancestors that I never gave DNA a
> thought until now.  This adds more flavor to the research that I love!
>  
> I noticed that I only have four "HVR1 differences from CRS" (mutations) in
> my mtDNA results posted on the database for Family Tree DNA while others
> have five, six and more.  What does this indicate?  I understand that the
> CRS is used as a basis for comparison with mtDNA and that, in my case, I
> have four instances of differences from the CRS.......what I do not
> understand is......is this a good thing?  :-)   Am I trying to read too much
> into this?  
>  
> Thanks again, Victor, for your help......and thanks to the other members of
> this group who have helped me!  I appreciate it!
>  
> Josie in San Antonio
>   _____  
>