Surname or DNA Switch

A great blog post written by Maurice Gleeson almost a year ago calling for the term 'NPE' (Non-paternity Event or Not Parent Expected) to be referred to as Surname or DNA Switch, 'SDS'. The blog is great at explaining the possibilities and probabilities of this occurring.

For a Doherty tester born in 1950 there is 31 – 52.5% of a SDS occurring in their patriline across 37 generations to what is believed to be the surname progenitor or common ancestor of 850 – 900 AD. (continue reading for the math involved).

His probability calculations on this occurring is based on several conservative estimates from various studies of 1 – 2% per generation. Note that he calculated this incorrectly with a correction in the comments.

For Ó Dochartaigh if we are to assume that Group 1 testers descend from the progenitor of the first man to earn the title "Dochartaig" then the testers are estimated to converge at around 850-900 AD.

If we have a male Doherty (or variant spelling) tester born around 1950 then, on average assuming 30 years per generation, there would be approximately 1,100 years to this progenitor or ~37 generations.

So, the calculation for a 1% per generation would be:

0.99 ^ 37 = 0.6894

which is stating there is a 68.9% chance this Doherty tester DID NOT HAVE AN SDS on his line, or, in other terms, there is a 31% chance this tester's patriline experienced an SDS.

Same calculation for 2% would be:

0.98 ^ 37 = 0.4735 or 47.4% of no SDS, in other terms 52.6% probability of a SDS occurring across 37 generations.

So, a male tester born in 1950 would have a 31 – 52.5% chance of a Surname or DNA Switch on their patriline across 37 generations or approximately back to 850 AD when Group 1 Dohertys converge on a common ancestor. Please keep in mind regardless of this happening on one's patriline it should not diminish your identity as a Doherty. If you are born a Doherty then you are a Doherty, plain and simple.

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The O'Doherty Heritage website is designed to be your place for all things Ó Dochartaigh. Whether your name or ancestry is from a Doherty, Dougherty, Daugherty, Docherty, Dogherty, if your family used another spelling of our name, or if you're from one of our related Donegal families, such as the McDevitt/McDaid, McBride, McConlogue, or others–we welcome you to the O'Doherty Heritage website!

The vision of the O'Doherty Heritage is "Telling our story, uniting us together." This was the same vision of our ancestors, the Ó Dochartaigh patriarchs, when they established our Clan motto, Ár nDúthchas–which means 'Our Heritage'. Below are the goals of O'Doherty Heritage:


  • Share our heritage with our entire Clan

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  • Curate a digital Clan genealogy research library


  • Encourage quality genealogical research

  • Promote clan identity and pride in our heritage

  • Facilitate communication amongst our clanfolk


This site is curated by Will Dougherty III under the direction of the Association of Ó Dochartaighs. ©2020 O'Doherty Heritage. Site Policies. I'd love to hear your thoughts on the site, answer your questions, or see you get involved! Email