Monday, July 6, 2015

A Brief History of ySNPs: Part 1

This is the first of a series of posts leading into how you can interpret NextGen ySNP results such as BIG Y and similar tests of discovery. These posts will not necessarily be written in chronological order. Sometimes when traveling through prehistory one loses an orientation to the currently accepted time continuum. I may retroactively renumber the parts of this series for those of you who prefer to have your information provided in a linear format.

We now estimate human ySNPs have been around about 338,000 years. This series is going to focus on only the last 4,000 years of one particular branch of the human ySNP tree. That is as far as I can now get my genetic periscope to come into at least a fuzzy focus. For reasons that are intuitive to some of you and chauvinistic to others, I am focusing on the journey of my own ySNPs. 


Many of you already know that a vast majority of our DNA consists of atDNA -- from within our autosomes. Our atDNA is incredibly helpful as we attempt to sort out family relationships within the last few generations. However, half of the atDNA information recorded in the cells of family is lost with each passing generation. We might say that atDNA information has a half-life of one generation. After five generations have passed, we can only interpret intelligently about three percent of the original information. This is why documenting the atDNA of the members of the oldest generation of your family should always be a high testing priority. Once they are gone, half of the family history information recorded in their autosomes is buried with them.


The information contained in our xDNA is similarly unreliable after a few generations and its quirky inheritance pattern adds additional twists to those who try to read it.

Celibate DNA

Those of us interested in using DNA to trace deep ancestry are left with celibate DNA. Celibate DNA is DNA that is passed down from generation to generation with the direct involvement of the parent of a single gender. It is not remixed with each inheritance event. Generation after generation it is copied with very few errors .


Mitochondrial DNA is a celibate DNA. It provides a window back into deep ancestry of our maternal lines. All of us carry a copy given to us by our mothers documenting the descent of our umbilical line back to mtEve. However, the story it can provide is limited to what can be recorded by using only one of 4 characters (i.e., A, C, G or T) at each of 16,569 locations. In other words it is a very short book. 


The Y-chromosome is the smallest of the 46 human chromosomes. Only men have one. However, with almost sixty million locations to record an A, C, G or T, its capacity to record its journey through time is vastly greater. It is a book that has about 3,600 pages for each page in the mitochondrial portion of our internal cellular libraries. As a result it can teach us much both about our human journey through prehistory and our families in the genealogical era.


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