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McCool Name Study - DNA Testing

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Surnames/tags: McCool McCoole
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DNA testing is a crucial part of the McCool Name Study. Neither WikiTree nor this project endorses any testing vendor; they all have unique strengths. However, for purposes of this study, we will focus on comparing matches from autosomal DNA tests from AncestryDNA, and Y chromosome tests from FamilyTreeDNA.


Autosomal DNA Testing (atDNA)

atDNA tests are the most common and least-expensive DNA tests. They are available to men and women, and provide information on matching to descendants of all of your lines. Everyone inherits half of their atDNA from each parent - and doesn't inherit their parents' other half. This holds true for siblings as well - but the atDNA half they inherit will be randomly different than each of their siblings. On average (but varies), two siblings inherit half of the same atDNA from their parents - and half of a different portion of atDNA from their parents.

There are several conclusions from this:

  • your atDNA is a subset of both of your parents; you inherit ALL of your DNA from them. If you can test both of them, you won't find any "new" atDNA by testing yourself.
  • atDNA is effectively "watered down" by about 50% in each generation - so test as many members of the oldest living generation as possible.
  • because atDNA is watered down every generation, it becomes less and less effective as we look back more generations.
  • if you can't test both of your parents, test yourself and as many siblings as possible.
  • remember that your aunts/uncles are your parents' siblings; the same rules apply to them.

Note that atDNA tests don't identify HOW you're related to a match, just that you ARE related. atDNA testing isn't perfect, and false positives (and negatives) become more and more likely below 16 or so centiMorgans (cM) in shared DNA amounts.

We focus on AncestryDNA matches for this project because they have the largest database of potential matches in the industry. We simply don't have time or the capability to try to compare large numbers of matches across tests from multiple testing vendors.

As of Nov 2020, we have a database of how about 1,100 McCool/McCoole AncestryDNA-tested descendants are (and are not) believed to be related to each other. Over time, we hope to incorporate much of that information into WikiTree.

How to include your atDNA test in the study

Regardless of your atDNA test vendor, please do the following:

  • Port your results to GEDmatch. This allows for some cross-vendor comparison of matches among those who choose to do so.
  • See below about how to link your WikiTree profile to your DNA test(s).

Y Chromosome Testing

Men inherit an X chromosome from their mother and a Y chromosome from their father. Women inherit an X chromosome from each parent (and don't inherit a Y). For this reason, Y-DNA tests are available only to men.

The Y-chromosome is passed down from father to sons, virtually unchanged (except for minor mutations) for thousands of years. By comparing the Y-chromosome of a tested man with all of the other tested men in a vendor's database, the vendor can determine which men share a paternal-line ancestor (son to father to grandfather) in a genealogically-meaningful timeframe. FamilyTreeDNA (FTDNA) has by far the largest database of tested men, so they are our vendor of choice for Y-DNA.

There are two types of Y-DNA tests: STR and SNP.

STR Tests

Without going into depth, STR tests examine a small number of areas on the Y chromosome that change somewhat frequently to determine if two men share a common paternal-line ancestor. FTDNA currently sells a Y-37 test (that examines 37 small areas of the Y chromosome) and a Y-111 test (examines 111 areas - including the same 37). Y-67, Y-25, and Y-12 tests are no longer available for purchase, but results from the older tests can be compared to the remaining tests. A Y-67 is a subset of the Y-111, and so on.

Two men are said to "match" if they have identical results at most of the tested areas. A genetic distance (GD) of 0 shows they match at all tested areas; a GD of 1 shows they have a single variation in one area, and so on. To match at Y-37, tested men must have a GD of 0 to 3 in those 37 areas. To match at Y-111, tested men must have a GD of 0 to 10 in those 111 areas.

Anyone who matches at any level on an STR test is "proven" to share a common paternal-line ancestor with their match. Theoretically, a lower GD means the relationship is closer. However, STRs can change back and forth, reverting to previous values. So while an STR match definitely proves a common paternal-line ancestor, it's not always clear how long ago that common ancestor lived. That being said, a GD of 2 at the Y-111 level is a much closer match than a GD of 2 at Y-37.

SNP Tests

There are large areas of the Y chromosome that are very stable and rarely change.

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