O'Dea/O'Day/Dee (DNA) Project

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About us

Statistical summary

The O'Dea/O'Day/Dee Surname Project now has 140 members, of whom 88 are males with Y-DNA results. Of these 88 men:
  • 58 are in the project because their surname is O'Dea or a variant:
    • 17 O'Days
    • 23 O'Deas
    • 14 Days
    • 1 Dea
    • 1 Dee
    • 1 Odey
    • 1 Godwin
    • 1 Deese;
  • four are in the project because they have known Day or O'Dea male-line ancestry but a surname/DNA switch in recent generations;
  • 22 are non-O'Deas, who are generally in the project because they have O'Dea ancestry other than through the patrilineal line; and
  • three have non-O'Dea surnames (Croke, Crow and Fitzgerald), but are in the project because they are genetically close to O'Deas (in some of these cases, the common ancestor may pre-date the adoption of surnames; in other cases, there may have been an early surname/DNA switch).
15 out of 88 men in the project have still not filled in the essential Paternal Ancestor Name information.

So far, 50 of the 66 men whose surname is O'Dea or a variant have given a placename in either the Paternal Ancestor Name or Country column, 42 in Ireland (including in Counties Clare, Cork, Galway, Limerick, Mayo and Tipperary), two (known relatives) in Wales, two in England, one in Scotland and three in the USA.

If you are not a member of this project and would like to join, click the Background tab above and scroll down to the "how to participate" section.


The objective of a suname DNA project is to reconcile the surname history in the ancient annals, the recent family trees of living bearers of the surname and the DNA results showing project members' places in the genetic tree of mankind.

The DNA analysis is based on the Y-chromosome, which descends from father to son like the surname. Autosomal DNA, which comes equally from both parents, from all four grandparents, etc., is also analysed by FamilyTreeDNA and its competitors (some of them much bigger and better-known than FTDNA). However, every segment of autosomal DNA ultimately came to some male ancestor from his mother, so it is of very little use to surname studies like the present O'Dea study.

The genetic tree of mankind was first drawn up around the turn of the millenium as this Simplified Tree of Y-Chromosome Haplogroups, dividing men into 18 or so top level haplogroups (groups of men with similar DNA signatures) represented by the letters A to R. Men in different top level haplogroups have no common male line ancestry within the past 20,000 years. As of 4 December 2021, technological progress had allowed these 18 branches to be broken down into more than 50,000 sub-branches of the tree of mankind.

The older and cheaper products based on STR mutations (Y-DNA37, Y-DNA111, etc.)  can only predict a man's place on the tree of mankind; one of the newer products based on SNP mutations (SNP pack, Big Y-700, etc.) must then be purchased to confirm the man's place on the tree of mankind.

In an ideal world, all project members with O'Dea (or variant) patrilineal ancestry would have Big Y-700 results, but this still normally costs USD449. The price is generally reduced around Father's Day in the United States (the third Sunday of June), from mid-November to the end of the year, and during other sale periods. Currently, all bar one of the 88 males with STR results have bought Y-DNA37 or higher, but only 27 have bought Big Y-700 or its predecessors Big Y-500 or Big Y, of whom ten are from non-O'Dea male lines.

As Big Y-700 has become cheaper and more popular, FamilyTreeDNA is pulling back from providing SNP packs and single SNP tests. Those who are curious as to exactly where they fit into the Tree of Mankind but who find the cost of Big Y-700 too much are encouraged to order relevant single SNP tests from YSEQ.

Identifying O'Dea-specific SNP mutations will in general require a second O'Dea (or variant) from the relevant subgroup on the results page to purchase Big Y-700. With any luck, he will share one of the novel variants of the existing Big Y-700 subject, which will then be deemed to be a new SNP and will be added to the tree of mankind.

Most ancient surnames have multiple genetic origins, either due to independent adoption of the surname by different men early in the surname era or due to more recent surname/DNA switches. With a documented history going back around a millenium, O'Dea is no different.

I have divided the 66 members with a patrilineal O'Dea connection into 32 subgroups, based on my analysis of their STR and SNP results, with a residual group for the 22 men with O'Dea ancestry other than through the patrilineal line.

Click here to open the results page in a separate window.

The history of the O'Dea surname would lead one to expect most O'Dea men, and certainly those with roots in County Clare, to have the easily identified Irish Type III or Dalcassian Y-DNA signature, characterised by the R-L226 SNP mutation. Not surprisingly, this accounts for over half of the O'Dea men in the project (36 out of the 66 with Y-DNA results; 7 out of the 16 with Big Y results). The subgroups are colour-coded, with all the Dalcassian subgroups having headers in coral or orange.

If your kit number appears in one of these coral-coloured subgroups, then you should also join the project set up at Family Tree DNA for the R-L226 Haplogroup if you have not already done so.

A significant minority of the project members with O'Dea and similar surnames are not Dalcassians (27 out of the 61 with Y-DNA results; 9 out of the 16 with Big Y results). Their surnames may originally have been Day or another variant, or there may have been a surname/DNA switch in their distant ancestry.

The Dalcassian O'Deas

All men with the Dalcassian Y-DNA signature share a common male line ancestor around 2000 years ago. Surname usage in Ireland dates only from around 1000 years ago. So there are many different surnames within the Dalcassian clan.

Our thanks are due to Dennis Wright, Dennis O'Brien and Robert Casey, who administer the R-L226  project, for all their advice on interpreting O'Dea Y-DNA results.

Dennis Wright has now identified well over a thousand SNP mutations which are found only in subgroups of Dalcassian men. These have all been given labels of the form R-DCxxxx, with DC indicating Dal gCais.

Even among the Dalcassian O'Deas, there appear to be several different genetic origins pre-dating the surname era. I will refer to the main branches as the Real O'Deas - who may have had a Dalcassian patrilineal ancestor using the O'Dea surname before the Battle of Dysert O'Dea in 1318  - and the Royal O'Deas.

An early objective of the project was to identify an O'Dea-specific SNP within the R-L226 subgroup.

The Royal O'Deas

Five members of the project are confirmed, or almost certain, to have the R-BY5212+ SNP mutation. This was previously believed to have occurred in a direct descendant of Brian Boru.

History and folklore tell us that the O'Briens and O'Deas fostered each other's children through the centuries.

It appears that at some point, probably after 1318, a boy with O'Brien DNA grew up with the O'Dea surname, possibly as a result of fosterage.

Three of the Royal O'Deas have Big Y results. All three are R-BY5212+ (as are many O'Briens), but only two of the three are R-DC1542+. Hence, the first man to have the R-DC1542+ SNP mutation was already using the surname O'Dea (in place of O'Brien).

It is highly recommended that the other two men in this subgroup (kits 363020 and 836494) order the single SNP tests for R-DC1542 from YSEQ.

The Real O'Deas

The most recent confirmed SNPs of the other Dalcassian project members are either a SNP common to all, or almost all, Dalcassian men (R-L226 or R-FGC5660), or one of the SNPs from the more recent R-DC135 branch of the haplotree. The surnames associated with R-DC135 can be seen here in The Big Tree and include Croke, Crow, Crowe, Fogerty, Martin and Woods in addition to O'Dea. Denis Wright of the R-L226 project estimated on 27 February 2020 that the R-DC135 mutation occurred around 1018AD (with a large margin of error).

This leaves it unclear which is older - the O'Dea surname or the R-DC135 mutation:
  • the first man to use the O'Dea surname may already have had the R-DC135 mutation, and some of his R-DC135+ cousins may have adopted different surnames when surnames were first introduced; or
  • the first man to have the R-DC135 mutation may already have been using the O'Dea surname, and some of his descendants may have had surname/DNA switches.
On the one hand, the wide spread of surnames under R-DC135 on The Big Tree suggests that the former possibility is more likely, and hence that R-DC135 predates the adoption of any surnames.

On the other hand, a Dalcassian O'Dea (kit B317480, who is not a close STR match to the Royal O'Deas) has recently tested negative for R-DC135, which suggests that the latter possibility is more likely. The R-DC135- result also opens up two further possibilities:
  • the O'Dea surname may have been adopted independently by two Dalcassian men, one R-DC135+ and the other R-DC135-; or
  • there may have been another surname/DNA switch to O'Dea from another undetermined surname in either the R-DC135+ or the R-DC135- branch of the haplotree.
Hence, it now appears that there may be not two but three separate origins of the O'Dea surname within the Dalcassian dynasty.

Within the project, we have a Day (with roots in Limerick) and an O'Dea (with roots in Clare) whose Big Y results have confirmed that they are R-DC135+, but have not found any SNP more recent than DC135.

The Croke, Crow and Crowe surnames not only have their own sub-branch of R-DC135, namely R-DC134, but R-DC134 also has two sub-branches (identified at FTDNA but not yet in The Big Tree), namely R-DC373 (Croke) and R-DC430 (Crow).

In due course, it is highly likely that a similar sub-branch and similar surname-specific SNP mutations will be identified for the Real O'Deas.

The New Zealand O'Deas (organisers of the March 2020 O'Dea gathering in New Plymouth), represented by kit 778222, are certainly Dalcassian, but it is impossible to say with just a Y-DNA37 result and just Minimum access given to the project administrators, whether they are more likely to be R-DC135+ or R-DC135-.

The non-Dalcassian O'Deas

The terminal SNPs identified for most of the O'Dea lineages are all in the ancient high level haplogroup R1b or R-M269 (along with the Dalcassians), but there are also 2 men in haplogroup E, one in haplogroup G, three in haplogroup I and two in haplogroup J.

The most recent confirmed SNPs for men in the non-Dalcassian subgroups include E-BY4507, I-L233, J-FT29852, R-BY3274, R-Z16270 (Godwin), R-BY105798, R-BY23818, R-BY70436, R-A6138, R-Z16435 and its descendant R-BY56625, R-BY56052 and R-L48.

Paddy Waldron
O'Dea/O'Day/Dee Project

Last updated 15 March 2020.

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