Some people eat, sleep and chew gum, I do genealogy and write...

Monday, October 30, 2017

Pedigree Collapse, Binary Progression, and DNA: Part Two

The structure of the DNA double helix. (Public domain photo / NASA)
A paradox occurs when an absurd or self-contradictory statement or proposition, is investigated or explained and then proves to be well founded or true. Pedigree collapse is paradoxical. You would expect that the number of your ancestors would continue to increase exponentially. But the reality is that they don't. As I mentioned in the first installment of this series, ultimately, the idea of pedigree collapse postulates that all humans are descendants of a single person or perhaps a small group of people in Africa. What is becoming evident by the DNA studies is that presently all humans are humans by virtue of their shared DNA. Here are a few articles discussing this issue.
What does this have to do with genealogy? Ultimately, genealogy is based on written records and the existence of those records. DNA, on the other hand, is based on random combinations of genes in our chromosomes. The differences between the two lie in the First Rule of Genealogy: "When the baby was born, the mother was there." In genealogy, the implications of this Rule are far-reaching. Essentially, mothers can be identified with a greater degree of certainty than fathers. 

Here is a simple example of the problem.

Look at your pedigree chart. It lists those people who your inherited genealogy or your own research are shown as biologically related.  But how do you know that the information you have on your chart is accurate? The answer is that you don't. Your conclusions are based on written records that may or may not have been accurate. For example, you may believe you are related to your known parents. You may even have a birth certificate to "prove" that relationship. But a DNA test may reveal that your named father is not your biological father and the same thing goes for your mother also. What about all those generations of records where you do not have a DNA test to support a biological relationship? What about all those children born out of wedlock or recorded marriages? How many of these relationships were culturally hidden?

One thing is certain. You get a random mix of genes from your parents. See "Relatedness." Another fact is evident, DNA testing results can disturb assume relationships. See "Couple discover they are siblings: Child courts blamed after strangers fall in love, have a son - and then find out they are half-brother and sister." When you undertake the journey of discovering your ancestry, you are entering a world where marriage between closely related people is not only possible but in the case of European royalty and other historically related groups of people, common. 

From another standpoint, identical twins do not have identical DNA. See "Identical Twins' Genes Are Not Identical." Before extensive DNA or genome sequencing became available, it was commonly believed that identical twins had identical DNA. But this has more recently been shown to be an inaccurate conclusion. 

So what is the "real" relationship between genealogical research and DNA? Both approaches provide information about a similar subject: relationships between humans. Both have limitations. Ultimately, DNA testing relies on statistical analysis. See. for example, "On statistical analysis of forensic DNA: Theory, methods and computer programs." When a genealogical DNA test is interpreted, the results are expressed as a statistical percentage correspondence or an "estimate." These ethnicity estimates are often combined with reports of DNA matches of varying percentages of DNA. These percentages are, for the most part, based on very small percentages of "shared" DNA. Unfortunately, a large percentage of these matched people do not have supporting family trees that allow an actual relationship to be established. In other words, even when the DNA matched person has an online family tree, there is no apparent genealogical relationship evident. Without a carefully documented and evaluated genealogically researched family tree, DNA testing is nothing more than an interesting estimate. 

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