I'm debating whether I should take a DNA test or not. I had a distant relative ask me if I've taken a DNA test because he thought it would help tie our families together. But I was under the impression that unless you share the same male or female line, it wouldn't be useful. Since the relative and I don't share the same last name, and more importantly, don't share the same direct male line, I felt it wouldn't be useful. But I wasn't sure.
So after studying the various tests, I've come to the understanding that there are generally three basic types of tests available - one for the male line, one for the female line, and one for general DNA analysis. The test for the male line is especially useful because others having the same male line can be identified. This type of test is called a Y-DNA test because it looks at the Y chromosome. Since women do not inherit a Y chromosome, they have to rely on their father's or brother's Y-DNA test to learn about their own male lines.
Both men and women inherit an X chromosome, but an X-DNA test is usually not provided. It turns out that it more complicated to trace the origin of X chromosome DNA, since both men and women pass it down, men get it from their mother, and women get it from both their father and mother. More commonly, the female line is tested using what is called mitochondrial (mtDNA) testing. Both men and women inherit mitochondrial DNA and can learn about their maternal lines using this test.
The third type of test commonly used is an autosomal test. This test looks at the remaining chromosomes, which are not X or Y. These tests are used to identify the general heritage mix that you have and can be helpful in proving a fairly close relationship between two people. It is not possible to identify a particular line as you can with Y-DNA and mtDNA.
So, for proving genealogical lines, the Y-DNA and mtDNA tests are the best. They can be used to positively identify paternal and maternal lines, respectively, and can be used to prove common ancestry between people. Unfortunately, these tests are limited to the maternal and paternal lines which excludes most of the family tree.
The autosomal test is interesting because it includes DNA from the entire family tree. Its weakness is that you can't know how the DNA was inherited. Comparisons can be made between individuals to determine a probability of relationship, but it doesn't prove how they are related. Even if you suspect that two individuals share a common ancestor, and the autosomal tests show a relationship, the two may actually have a different relationship than the one you suspect, and the suspected link may or may not even exist.
Its interesting to consider that relatives do not necessarily share any autosomal DNA (atDNA) at all. For example, two cousins could inherit entirely different sets of atDNA from their common grandparents - still cousins, just no DNA in common. The odds are extremely slim, but when you start talking about distant cousins, it becomes more likely. On average, second cousins share just over 3% of their atDNA.
It is also interesting to consider that you may have absolutely no atDNA in common with a known ancestor. In fact, your genetic family tree is considerably smaller than your genealogical family tree, because many ancestors do not contribute to your DNA makeup. It reminds me of pedigree collapse, the situation that occurs as your ancestry goes farther back, where people start showing up more than once in your ancestry.Your actual family tree is much smaller than your theoretical family tree. I wonder how this pedigree collapse and genetic tree collapse are related. It would seem that with an ancestor showing up in your tree multiple times, that it would be more likely that you would actually inherit DNA from him or her.
In any case, I don't have a reason for a Y-DNA test, since I'm pretty confident about my paternal heritage. And I'm not convinced that the atDNA test would help extend my genealogy. Perhaps it would give a few tips regarding general heritage.
But the mtDNA might be interesting. My direct maternal line ends with my 2nd great grandmother and her origins have puzzled me for years. Perhaps submitting mtDNA results to a database would yield some potential relationships. I think its going to take a bit more consideration and understanding. Hopefully what I have uncovered so far is correct. Please understand that I am certainly no expert in the subject. But I do hope that it helps to explain things some.
Bob Mesenbrink
July 8, 2015
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