A good blog on Genetic Genealogy basics


As I said before, I am still trying to fully understand the basics of genetic genealogy. After digging through some blogs, I find the clearest voice on these matters to be Steve Handy, a grad student at UIC who wrote a blog called "DNA Matches."

Here are some relevant posts and my takeaways:

1. "Understanding Autosomal DNA Testing"

Takeaways: An autosomal DNA test (AncestryDNA, etc.) only "reflects relationships within a genealogical time frame," roughly within the last 300 years. Any further back in time and the process of recombination — parents' genetic material being broken down and reformed at conception — ensures that the amout of DNA we inherit from those distant ancestors is too small to be distinguishable.

DNA matches reflecting common ancestry are called IBD (identity by descent), while DNA matches due to coincidence are called IBS (identity by state). Using the centiMorgan to measure two people's matching DNA segments, anything above 10 cM reflects probable common ancestry and anything above 20 cM is definitive common ancestry.

2. "Understanding Correlations and Debunking Misconceptions In DNA Genealogy"

Takeaways: You cannot determine your ancestors' race, religion, or ethnicity from DNA. Even I am guilty of thinking that a DNA test can show all my "ethnicities," but "ethnicity" is a social construct.

All these DNA tests are measuring "Ancestry Informative Markers" (AIMs) that are present in high frequencies in particular populations. However, not every correlation is correct — not every genetic marker is indicative of only one particular population, but could appear among multiple groups.

A personal note — I am a Jew, and most of my mother's family are Ashkenazi Jews. I feel uncomfortable when I hear DNA tests saying that someone is a percentage "Jewish," since it evokes the centuries of anti-Semitic claims that Jews could be distinguished by physical traits. What modern DNA tests mean by Ashkenazi Jewish percentage is simply what percentage of AIMs come from a very endogamous population that developed from centuries of intermarriage, which in turn reflect strict religious laws and widespread discrimination and persecution.

3. "Understanding BGA Testing"

Takeaways: All the tests that show your percentages coming from different populations are called biogeographical analysis (BGA), and the mathematical process to get the results is called Principal Component Analysis (PCA). Scientists have not accepted the validity of BGA tests, and we have to accept that the results are probablities, rather than fact.

Handy writes:
"a BGA test is attempting to infer information from DNA that DNA doesn't define. An ancestor's original location can be anywhere. DNA simply doesn't reflect or store that type of information. From the frequency (or concentration) of those DNA markers in each population, we are making an inference which could be right or wrong."
Another word for the DNA percentages are "population sets," but again, they only show which DNA markers match reference populations. They are not clear-cut indicators of ancestry.

4. "Autosomal DNA Testing: Recombination"

Takeaways: Recombination is a random, unbiased process. I received half of my genes from each parent, but I cannot assume that the AIMs I received from each parent reflects the total proportions of their AIMs.

Also, people from endogamous populations have very homogenous gene pools, and recombination ensures that they end up with similar smaller segments. That is why, for example, Ashkenazi Jews may appear from their DNA results to be closer relatives than they actually are.

The very last post by Handy shows how he broke a genetic "brick wall" and found his Ridgway ancestors from Alabama. It demonstrates how African-Americans can use DNA testing to learn about their antebellum Southern white ancestors, who largely tended to be slave owners or involved with slavery.

I can only imagine the emotional impact of learning about your particular ancestors who were slaves or fathered by slave owners. My own African ancestors probably arrived in Colombia long before the "genealogical time frame" found in autosomal DNA, but maybe I will gain more insights into their lives someday. 

Questions? Comments? Please email me at ruedafingerhut [at] gmail.com

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