• According to several different studies, the average incidence of SDSs seems to be about 1-2% per generation.
• So, if we allow 25 years per generation and hence 4 generations in 100 yrs, that will give us 40 generations in 1000 years.
• multiply that by 1-2% per generation and that gives us a 33-55% chance of an SDS on any ancestral line going back 40 generations [for these corrected calculations see the first comment in the Comments section below]
• If we go back along the father’s father’s father’s line, then there is a 33-55% chance that your Y-DNA does not go back to the person who originated that surname.
• If we allow 33 years per generation (and hence 3 generations per 100 years), then the chances of an SDS in the past 1000 years is 28-49% on each ancestral line [for these corrected calculations see the first comment in the Comments section below]
• Whether you choose the 33-55% option or the 28-49% option, about half of us will not share the same DNA as the man who first held our surname … but half of us will.

It was not unusual for the servants, soldiers, vassals, tenants, or slaves of a clan to take (or be given) the name of their chief. This was a sign of respect for the chief but may also have conferred protection on the bearers of the surname. As a result, one would expect surnames that were associated with powerful clans to have many different Y-DNA signatures. And conversely, surnames that were associated with non-powerful clans and septs would have relatively few Y-DNA signatures. Surnames that might demonstrate this kind of pattern of multiple Y-DNA signatures associated with a single surname might include: O’Brien, Kennedy, O’Neill, O’Donnell, etc

This is an example of a surname-switch SDS. It means that the father who is doing the child-rearing will raise a son with the same Y-DNA as himself but with a different surname.

The Past was a different country – different customs, different reasons for Surname  or DNA Switches (SDS)

A similar situation arose with the Trans-Atlantic Slave Trade, where many enslaved people were given the surname of the Plantation Owner. Thus many African Americans today will carry the name of the slave owner of their ancestors (i.e. SDS present), and some will carry the Y-DNA of that slave owner (i.e. no SDS present), or his family (possible SDS), or the overseers he employed on his plantation (SDS present).

Adoption / Fostering / Guardianship

Fostering on the other hand, did not frequently result in a surname switch. Under the Brehon Law of fosterage, children were brought up by relatives who were usually within the 5th degree of kinship (e.g. great uncle, 1st cousin once removed). However, the child did not adopt the family name of his foster family. Instead he retained his own family name and thus there would have been no SDS as a result.

Similarly, with modern-day fostering, the child in question does not usually take on the name of the foster family i.e. there is no SDS.

It would not be unusual for the young children of a woman recently remarried, to adopt the surname of her new husband. This results in the child-rearing father (i.e. the second husband) having a son (i.e. stepson) with the same surname as himself but with different Y-DNA (i.e. from the first husband).

“Widowed and fatherless”

Sometimes it was stipulated that a man would have to change his name in order to marry the daughter of a wealthy man to ensure that the family name was carried on. A good example is the Pennington family of Muncaster. There are two recent generations where the husband had to change his surname to Pennington as a condition of marriage. Thus, there have been two successive switches in the DNA associated with the Pennington surname along the same direct male line of descent.

A similar condition arises where the prospective heir has to change his name as a condition of inheritance – “You won’t inherit a penny, my boy, unless you change your surname to Sidebottom”.

“Sidebottom, grandfather? … Seriously?!”

Taking wife’s name upon marriage (because of her higher social status)

In times past, it was not unusual for a man, upon marriage, to take the name of his wife if she was considered to be of higher social status. A famous example is that of Oliver Cromwell. His real name (perhaps we could say his maiden name) was Oliver Williams. His great-great-grandfather, Morgan Williams, married Katherine Cromwell in 1497. She was sister of Thomas Cromwell, Henry VIII’s chief minister, who was famously promoted to the earldom of Essex but later executed in 1540 when he fell from the King’s favour.

In ancient Irish culture, apparently it was customary to offer one’s wife to visiting dignitaries for “entertainment”. The custom is still practiced in certain cultures today. Thus the dignitary’s Y-DNA would become associated with any offspring of the union, but the child could be raised with the surname of the woman’s husband.

Why was such a custom practiced? A child by a powerful man would give protection to a family and also had rights to succession under Brehon Law. A good example is that of Mathew (Fear Dorcha) O’Neill(1520-1558), the illegitimate son of Conn O’Neill. He was accepted by Conn as his natural son and was made his heir. However, Shane (Mathew’s half-brother) contested his right of inheritance and Shane’s men eventually killed Mathew, but not before Mathew had a son, Hugh O’Neill, who later went on to become The Great Earl and Clan Chief of the O’Neill. On this occasion the Y-DNA and surname remained intact (no SDS) but in other cases, the surname of the father who reared the dignitary’s child may have been retained (i.e. SDS present).

There was also an associated custom of “Naming” children on a wife’s deathbed (i.e. she would reveal which of her children were the offspring of famous people). Apparently, as she lay dying, the wife would call her husband into the bedroom and something like the following conversation would ensue:

WIFE:           Husband dear, I need to have a word with you.
HUSBAND:   My dearest wife, what do you want?
WIFE:            Well … you know our eldest child?
HUSBAND:     Of course I do … sure isn’t he my son?!
WIFE:              Well … that’s what I want to talk to you about …

This sort of conversation does not happen very often today (or does it??), but it was relatively commonplace 500+ years ago.

In many ways, the old Brehon Law system practised by the mediaeval Irish, as well as their social attitudes, were more advanced and less restrictive than the laws we have today, especially in relation to the rights of women. There was a completely different concept of infidelity under Brehon Law. And such thinking is quite foreign to our modern ears.

How common infidelity was in medieval society is open to question. But if it was commonplace, and in addition married couples practiced regular sexual relations, then it must have been very difficult for a man to know which children were his and which were someone else’s. Maybe they played guessing games at medieaval cocktail parties.

Attitudes to infidelity have varied with time and place. In Shakespearean times, “cuckolding” was the subject of derision and the cuckolded husband would be mocked. In Victorian and more modern times, infidelity carried a stigma and was taboo. As a result, it would probably be kept secret and remain undetected. Husbands would be blissfully unaware that their child carried another man’s DNA.

Infidelity usually goes undetected, but sometimes there may be clues in the genealogical records. I came across a note written by the parish priest in the margin of an 1890 baptism record which read: this is not the husband’s child – he has been working on the docks in Liverpool for 18 months.

“Not tonight, Joseph”

In more recent times, and certainly since the introduction of Victorian morality, the illegitimate offspring of any “out of wedlock” encounter would usually be given the maiden name of the mother. However, this was not always the case and I have seen illegitimate children christened with their biological father’s surname (in which case, no SDS is present).

Girl Power under Brehon Law

Anglicisation of surname

It was common for surnames to be anglicised in Ireland. And as a result there are some unlikely genetic associations between different surnames. The surname Green was at times an anglicised version of Hooney or Fahy, both of which are derived for an Irish word for the colour green.

Surname change to match the prevailing social circumstances 

Sometimes people would change their name to suit their circumstances. A lot of Jewish immigrants to the US changed their name to make it sound more “English” and thus avoid discrimination. Similarly Catholics living in a predominantly Protestant community might change their name to “fit in”.

There is a popular belief that a lot of surnames were misspelt or assigned as people entered the US via Ellis Island but this has been contested for a variety of reasons. Firstly, passenger lists were not created in Ellis Island but at the port of departure of the immigrant i.e. in his or her native country, probably by a native speaker of the immigrant’s language. The inspector at Ellis Island did not write down the immigrant’s name – he worked from the passenger list.

Abandonment by husband

Occasionally we hear stories of babies being swapped at the hospital and the mother going home with the wrong baby. Several recent cases have come to light with the help of DNA testing.

More modern causes

This is not an exhaustive list of the causes of Surname or DNA Switches, so if you can think of other examples (and I am sure there will be many), please leave a comment below.

But hopefully it will be abundantly clear from the above, why you only have a roughly 50:50 chance that your Y-DNA goes all the way back to the person who originated your surname some 1000 years ago (or thereabouts). And why, as you go back along each ancestral line in your family tree, there are likely to be 3-parent and 4-parent families. In other words, one particular family may consist of a genetic father as well as a genealogical one. And some families (e.g. where an adoption has taken place) will consist of two genetic parents and two genealogical parents.

As a result, we all have two types of family tree superimposed on each other – one purely genetic, one purely genealogical. But both equally valid in terms of our family history. Both types of ancestor (genetic & genealogical) have contributed to who their offspring became … and who we are today.