Saturday, January 20, 2018

Y Chromosome DNA In Genetic Genealogy; Jan 2018

the castle of Iníon Dubh, where I first located the trail of my McCain family


Y Chromosome DNA In Genetic Genealogy


This is a brief introduction on Genetic Genealogy using the Y chromosome. The Y chromosome (Y-DNA) is passed from father to son only, in a direct male line, and this makes it the best test for surname studies. A Y-DNA test is for men only. Women do not carry the Y chromosome.  Women use a male proxy in the line they are researching to obtain a Y-DNA sample.  Men who are researching a non direct paternal line must also use a male proxy from the line they are researching. 

There are two types of tests used with Y-DNA.  There is a STR (short tandem repeat) test and a SNP (single nucleotide polymorphism) test.   STR tests are used to find genetic matches in a direct paternal line.  These matches can be recent to distant.  With a company like Family Tree DNA Ltd, this test comes in the form of a 11, 25, 37, 64, or 111 loci test.  Most participants use the 64 or 111 level as it provides more data.  The STR test will easily locate a paternal DNA match and give an estimate of the time to the most recent common male ancestor. 


One aspect of a Y-DNA STR test is that it allows you to confirm kinship to lines that you have no paper records to show a connection.  This allows the Y-DNA participant to use primary source records to all the families in his match group.  A family may have branches they did not even know of prior to testing.  As these matched families collect their data, a much better history of the family is revealed.  One letter from one family, one note on a census record, etc., can allow all the matched families to use the data with complete confidence. 

A wet day in County West Meath just prior to a Bean Sí incident

However, STR matches can have some issues in analysis.  Some STR loci will mutate back to, or close to, their original position.  This means two branches in a family may appear much closer (in time) to their most recent common ancestor.  This situation is called ‘convergence.’     

Conversely, if Joe sat under a UFO one night, he might have one STR mutation that shows a great distance to a shared common male ancestor, even to the point of Joe not being listed as a match at the lower level test.  At the higher level test, we can see Joe is a match. Poor ole Joe just had a unique mutation on that one STR locus. I have seen this happen several times in my consultant work. The issue is one STR locus mutated a greater distance than is normal. This is why the higher level STR tests are recommended.

In the early days of genetic testing, SNPs were dated thousands of years ago and showed basic ethnic and tribal connections.  In the past few years SNP testing has advanced greatly. Many new SNPs have been discovered and the number is growing. The new SNPs are younger in age and provide data that can be used in genetic genealogy.  SNPs are more stable than STRs.  The mutations are permanent and take place at a stable rate.  SNPs provide a method to expand on your STR test and match group and resolve any issues that come up.  


The data is becoming so detailed that the SNPs can differentiate between individual branches in a family.  So, even when there are no primary sources available, an advanced SNP test can ascertain how two branches of a family are related and locate the geographic point of origin.  As you explore down the SNP path, downstream to newer SNPs, you may find a SNP unique to one line in a family.  For example: this will allow you to tell the difference between a branch in a family from Porthall, Donegal, to a related branch in Corbally, north County Antrim.  Alas, with SNP test at this level, you had best bring your cheque book as the path can be a long one, but it is the ultimate tool for the genetic genealogist. 

R1b Y-DNA Haplogroup

 I work in the geographic area of the north of Ireland and western Scotland.   This includes the province of Ulster, the west Highlands and southern Hebrides, and the western Scottish Lowlands.  This population is remarkable homogeneous and many families share distant common ancestors.  Irish and Scottish families often benefit from advanced SNP testing.   


A summary: both STRs and SNPs are used in Y-DNA testing for genetic genealogy. Your Y-DNA test will include your STR results and matches, and a basic SNP Haplogroup.  This Haplogroup will show paternal ethnic origins and pre surname connections.   At this point, you will normally do more advanced SNP tests to discover your downstream (more recent in time) haplogroup.  This will allow you to confirm family branches, clan connections, and locate the geographic area associated with your family.  

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