After a trilogy dedicated to the use of old press to verify a family story through the concrete case of my ancestor François Mouyon, I propose to discover a genealogical investigation in two parts based on DNA.
Do you have an unknown ancestor in your family tree?
An untraceable biological relative, a blocked family branch, or a mystery that the archives cannot solve?
Today, genealogical DNA opens up new possibilities. Thanks to DNA tests, it is possible to identify genetic cousins and gradually trace back to a common ancestor.
But be careful: contrary to what one might think, a DNA test does not provide a direct answer. One must know how to interpret the results and apply an investigative method.
Can we find an unknown ancestor through DNA?
That is the question I have tried to answer through a real genealogical investigation.
Thanks to genealogy and DNA, I was able to uncover a previously unknown parallel branch of my family.
For this research, I use the DNA results from an autosomal test via the platforms MyHeritage and 23andMe.
Why use DNA in genealogy?
After having done my own DNA test, I suggested to my two parents that they also take one, with their agreement.
Why test your parents?
Because their results allow me to more precisely orient my DNA matches on the paternal or maternal side. Indeed, the percentage of DNA shared with a match is higher in the generation of my parents than in my own, which facilitates the identification and interpretation of genealogical links. It is also relevant to test uncles, aunts, brothers, sisters, cousins, or female cousins, as the segments of DNA passed down differ and generate new matches, helping to guide or confirm your research results.
This article shows you how to use DNA to find an ancestor. Want to know more about how DNA works? I refer you to the article from MyHeritage.
After taking a DNA test, many matches will appear in the list.
Start by analysing the matches with the highest percentage of DNA. You will find more information on how DNA matches work on the blog from MyHeritage.
In summary, testing your parents allows for better interpretation of your DNA matches by clearly distinguishing between the maternal and paternal branches.
How much DNA do we share with our family?
The graph below represents the approximate proportions of DNA shared among family members. Note that when there is a half relationship, the figure should be divided by two.
The graph below shows the amount of DNA shared.
It is observed that the percentages of DNA decrease very rapidly from one generation to the next.
To remember: the more distant the relationship, the more quickly the amount of shared DNA decreases.
Shared DNA: what do centimorgans (cM) mean?
Centimorgans (cM) measure the amount of DNA shared between two people. The higher this number, the closer the family relationship.
One centimorgan corresponds to a 1% probability that two genetic markers will be separated during the transmission of DNA from one generation to the next. This phenomenon, called recombination (or crossing-over), involves an exchange of DNA segments between the chromosomes inherited from each parent.
The number of centimorgans (cM) shared with a match refers to the total length of all identical DNA segments passed down from a common ancestor. The relationship will be closer if there are few long segments rather than many short segments, as long segments are less likely to be false positives. Indeed, segments can be identical by state (false positive) and not by descent.
In practice, it is mainly the long segments and the total in cM that help assess the reliability of a DNA match.
How many centimorgans do we share with our close relatives?
This diagram, created from data from the DNA Painter website, identifies the ranges of centimorgans typically observed according to the type of family relationship.
Graph of centimorgan sharing in a family, figures taken from the Shared cM Project version 4.0 (March 2020), Blaine Bettinger
Link between centimorgans (cM) and percentage of DNA
The table below links the percentage of shared DNA to centimorgans based on relationship.
Correspondence between cM and percentage of DNA based on relationship, data from DNA Painter
Thus, centimorgans provide a more accurate measure than percentage for estimating a family relationship.
How to convert cM to percentage of DNA?
To better understand this percentage through a bit of mathematics: the total length of the human autosomal genome is estimated to be about 7000 cM.
Indeed, a parent transmits on average 3480 cM to their child (range: 2250-3720 cM).
Thus: 3480 ÷ 7000 = 49.7% ≈ 50%, which corresponds to classical genetic theory.
This range was refined in version 4.0 of the Shared cM Project (March 2020) by Blaine Bettinger, based on over 59,000 observations of actual DNA matches, making the minimums more realistic.
Using the following formula, it is possible to perform the cM <> DNA percentage conversion.
Percentage of shared DNA = (shared cM / 7000)*100
Here is a simple step-by-step method to effectively utilise your DNA matches and trace back to a common ancestor.
Method: finding an ancestor through DNA in 5 steps
1. Understanding relationship
Whether you are using MyHeritage, 23andMe, or another DNA testing service, understanding the key concepts related to your matches is essential for effectively guiding your research.
Percentage of shared DNA, centimorgans (cM), and family tree are all valuable clues for identifying a relationship.
Tools generally provide an estimate of the family link, but this remains indicative. To refine the analysis, it is recommended to check if the match has created a family tree. This allows you to answer an essential question: what is the relationship between this person and me?
Once this initial estimate is made, it is necessary to go further by reconstructing the family trees.
2. Reconstructing the family trees
The second step is to reconstruct the two family trees.
Even if your match has already started one, it is often incomplete. Therefore, it is important to complete the trees as much as possible in order to identify a common ancestor.
3. Contacting DNA matches
Finally, an often underestimated element can make all the difference in a DNA investigation. Communicating with your matches on DNA platforms often helps to unlock situations and promote mutual assistance.
4. Identifying the common ancestor
Once the kinship is identified and your trees are created, it is relevant to find the position of your common ancestor(s) in your respective family trees. To do this, you can compare the two trees to spot a common ancestor.
However, in some cases, this approach is not enough and no common ancestor appears clearly. It is then necessary to change methods and broaden the search.
5. Exploring the archives (census records, military registers, and witnesses in the records)
What to do in case of a genealogical block?
Sometimes, identifying the common ancestor is not enough and the research seems to lead to a dead end. In these moments, it is essential to remember that archives do not always perfectly reflect reality.
Our biological ancestors do not necessarily correspond to those who appear officially in administrative documents. It is therefore possible that no common individual appears when comparing two family trees.
Rather than giving up, one should change approach and look for indirect points of convergence between the two lineages.
Several avenues can be explored:
Did both individuals live in the same city, the same street, or even at the same address at the same time?
Were they neighbours?
Did they have similar occupations or work in the same place?
Could they have known each other?
To answer these questions, it is essential to cross-reference sources:
Population censuses
Military records
Addresses mentioned in the records
Witnesses present at events (birth, marriage, death)
Photograph, FELLETIN 23, Creuse Carpet Manufacturing Weaving Workshop 1915 of Sergeant FIS 342nd to Albertine Foix, Delcampe
Departmental archives are the main tool for this type of research. Paid platforms like Filae can save time, even if not all censuses are available there.
It is also possible to use tools like SocFace, which uses automatic handwriting recognition to analyse census records from 1836 to 1936. However, these databases contain errors in the interpretation of names and surnames. It is therefore preferable to systematically refer to the information directly in the original registers.
A special case, the public assistance registers
When the parents of a person cannot be found and they are indicated as 'without parentage', it can be very useful to consult the public assistance registers.
These contain information about abandoned children (placed in a hospice), as well as those later recognised by their parents. These documents (birth registers, nursery or hospice records) can sometimes reveal valuable information, such as the name of the mother.
Example opposite of a hospice turntable. Babies were abandoned in a rotating cylinder in the wall of a hospice, allowing parents to anonymously leave their newborn.
Model, hospice turntable, Municipal Library of Lyon
Departmental and municipal archives are a real goldmine for this type of research.
It is also important to check if the common ancestor identified through DNA has previously faced this type of situation (abandoned child or recognised after birth). If this has happened once, it is possible that it has occurred again and that the person designated by the archives is not the true biological parent.
Conclusion: DNA, a key tool to overcome the limitations of archives
DNA has become an essential tool for genealogical research. When used properly, it allows us to go beyond the limits of archives and find unknown ancestors.
By combining method, rigor, and analysis of matches, anyone can today enrich their family tree and discover new family branches.
In a second part, we will see how this method allowed me to trace back concretely to an unknown family branch by cross-referencing DNA matches and archives.
Cover photograph: Charity Hospital, Dr Voron's department, Lyon, Delcampe