Interpreting DNA Results
By Angelo Coniglio
Recently, a reader had questions about DNA testing.
Q: My father, brother and I each had our DNA tested. My brother and I each were shown to match about 50% of our dad’s DNA. But when my brother and I compared our results, there’s only about a 40% match. We both have the same parents. Shouldn’t 100% of my DNA match my brother’s? Also, my results were interpreted by the test to show my ancestry as 75% Irish/British, with the remaining significant portion as Southern European. My brother’s also shows large portions of Irish/British and Southern European, however a percentage of his ancestry is shown as Middle Eastern. Again, we have the same parents. How can my brother have ancestry that I do not? ~ K. McConti, Amherst
A: Siblings’ DNA will match 100% (or nearly so) only for IDENTICAL TWINS, who formed from one sperm and one egg, and therefore each have exactly the same half of each parent’s DNA.
Neither of you got ALL the DNA of both parents. You each inherited half your DNA from each parent, so you got some DNA from your father that your brother didn’t, and he got some that you didn’t. The same is true of your shares of your mother’s DNA. On average, non-identical siblings show about a 50% match, but the percentage in individual cases may be somewhat higher or lower.
Regarding the ancestry part of your question: again, you each receive a different ‘sample’ of DNA from your parents. Your brother’s sample may have simply included some DNA from a distant ancestor from the Middle East, while the DNA you inherited did not contain that particular ancestor’s DNA.
Check the DNA test results for your father. Do they show Middle Eastern ancestry? If so, that’s where your brother’s Middle Eastern DNA came from. If not, if your mother is living, consider having her DNA tested. Her test could show a Middle Eastern connection, but if it doesn’t, remember, as technical as it may be, genealogic DNA analysis is still somewhat prone to error when estimating ancestral ethnicity.
Another common question: My DNA test results show a large number of individuals who are identified as “fifth to eighth cousins”, and even more as “distant cousins”. Other than identifying these folks as my “DNA relatives”, how does that help my research?
A: You may actually learn more about your ancestry from the more “distant” relatives.
For example, if you think about what your siblings or first cousins might know about your ancestry, they probably have no more insight than you do, or possibly less, if they’re not as “into” genealogy as you are. But say you have identified your ancestors back to and including every one of your great-great-great grandparents, both males and females (that is, your “3rd-great-grandparents”). You then have a detailed “family tree” or “pedigree” showing all their names, birth dates, etc. Any one of the couples forming that group would be five generations removed from you. A descendant of any set (e.g. a couple) of those 3rd-great-grandparents who is also five generations removed from them is your fourth cousin.
Now, if you find a “DNA relative” who is your fifth cousin, and he has compiled a family tree back to a common ancestral couple, who are his 4th-great-grandparents, they’re also yours. Those ancestors would be the parents, previously unknown to you, of one of your known ancestors, and so your “tree” would be extended back in time. Note that DNA testing would simply connect you to this fifth cousin. It would be his research that revealed further ancestors.
Coniglio is the author of the book The Lady of the Wheel, inspired by his Sicilian research. Order the paperback or the Kindle version at http://bit.ly/SicilianStory Coniglio’s web page at http://bit.ly/AFCGen has helpul hints on genealogic research. If you have genealogy questions, or would like him to lecture to your club or group, e-mail him at genealogytips@aol.com
