Using Y-chromosome DNA
DNA testing does not provide names and so is not a substitute for more traditional family history research techniques. However a genealogist can use DNA testing as another tool to answer some family history questions in the absence of other documents.
Some questions that might be answered by a Y-chromosome (Y-DNA) test include:
- Another man with the same surname lived near my ancestor. Were they related?
- I think my female ancestor had a lover (or a second husband). Can I determine which man fathered her son?
- My grandfather was adopted. I have a theory about who might be his father, can I prove it?
- Can I prove that he and his brother are sons of the same man?
Y-chromosome DNA (or Y-DNA) is a particular type of genetic material that is passed largely unchanged between a father and his son. As such, comparing the Y-DNA between two men can answer questions about whether they shared ancestors on the all-male line.
A female like myself who wants to answer the same sorts of questions needs to find a willing male close relative to be tested. I have ordered Y-DNA tests for my father, his mother’s nephew and also my mother’s brother in order to examine my nearest male lines.
In theory Y-DNA follows the path of surname inheritance and so finding a close Y-DNA match with another male who has the same surname is a good indicator that those men share an ancestor.
There are many reasons why a surname might not have been inherited along with Y-DNA. Geneticists refer to these as ‘non-paternity events’, when the father is either unknown or not the person commonly believed. A Y-DNA test could confirm a theory about paternity even when the possible father and son have different surnames.
To look for relationships between men long dead I need to find a living male descendant (down all-male lines) from each of them.
Sometimes that means stepping sideways – for example, researching a brother’s line if some man in the line had no male descendants. In other words, it is necessary to combine the use of more traditional family history research techniques with the new information offered by DNA.
A male with no clue about their father could take a Y-DNA test to learn which surnames occur most frequently amongst their genetic relatives and use these names as possible clues. (Alternatively a different DNA test – an autosomal test – might locate a cousin and that might help identify the father.)
I have also had serendipitous discoveries, when someone who tested with the same company was identified as a Y-DNA match with one of my family and we were able to identify the shared ancestor. Thus I have discovered previously unknown cousins, allowing an exchange of information about family history.
How does it work?
Every cell in your body has 23 pairs of chromosomes, inherited from your parents. The 23rd pair are the sex chromosomes – males have an X- and a Y-chromosome while females have two X-chromosomes. Note that only males have a Y-chromosome.
In each generation the Y- chromosome (or Y-DNA) of the father is copied (largely unchanged) in order to be passed on to his sons. However occasionally cells make a copying error.
Sometimes the number of repeats of a group of DNA ‘letters’ (called a short tandem repeat or STR) is increased or decreased.
The other mutation occurs only very rarely, when a single DNA ‘letter’ is miscopied (rather like a typo) – this is called a single nucleotide polymorphism (or SNP, pronounced ‘snip’). As both these changes are then transferred to future generations, the Y-DNA becomes rather like an audit trail, recording inheritance on the direct paternal line.
These mutations are illustrated in the diagram below. In the example Man 1 is the father of Man 2. When the Y-DNA of Man 1 is being copied, a SNP occurs when a ‘T’ is accidentally miscopied as an ‘A’. This change is inherited by Man 2 who then passes it on to his descendants.
Man 1 also has a segment of DNA with a short tandem repeat (STR) where the ‘letters’ CTA are repeated 5 times. We say that Man 1 has a repeat count (or allele) of 5 at that point. When that repeating section was copied for passing to Man 2, the number of repeats was increased – so 5 repeats became 6 repeats. By the time that DNA was re-copied and passed to their descendant Man 3, those 6 repeats have mutated to become 7 repeats. Note that Man 3 also continues to carry the ‘A’ variation of the SNP inherited by Man 2.
A genealogical Y-DNA test reports on certain STRs on the Y-chromosome. Rather than checking the whole chromosome, a male can order a Y-DNA test of certain useful sections of DNA (or markers) – currently available tests examine between 12 and 111 markers. When the repeat counts at those markers are compared against those of another male, a close match indicates that the two men share an ancestor on their all-male or patrilineal line. Depending on how closely they match, an estimate can be made about how many generations earlier their most recent common ancestor (or MRCA) probably lived.
Genealogists should test at least 37 markers. Anything less and it could be that an indicated shared ancestor lived many hundreds or even thousands of years ago.
Genetic genealogy tests also examine the SNPs (‘typos’) on the Y-chromosome. Such SNPs indicate the haplogroup – or where the male tested fits into the broad family tree of all men.
Which company to use?
The US company Family Tree DNA is the main company offering specific STR marker testing of the Y-chromosome with the required level of accuracy required by genealogists as well as having a large database of others’ Y-DNA results for comparison. This company offers tests of between 37 and 111 markers as well as tools to interpret the results. For under US$170 (at the time of writing) it is possible for males to test their Y-DNA with sufficient accuracy to determine whether two men are likely to share a common ancestor ‘within a genealogical timeframe’ and also receive an estimate about how many generations ago that shared ancestor lived.
You can save money by ordering the Y-DNA test through a surname-specific project. Projects are managed by knowledgeable volunteers who analyse the similar DNA of large groups of people in order to draw further conclusions. By looking at the DNA of enough people with the same surname, it may be possible to identify when a particular mutation occurred. For example you may learn that ‘all those with this particular mutation descend from one brother but those without that mutation descend from the other brother’.
Many projects focus on a common surname but there are also geographic and ethnic heritage projects. Joining projects is free and you can usually join any that might be of interest – such as for other surnames that occur frequently among your DNA matches! Once you discover your Y-DNA haplogroup, I recommend you also join the relevant haplogroup project – those matches share Y-DNA heritage, whether or not they share surnames.
Family Tree DNA
Projects on Family Tree DNA
Webinars on Family Tree DNA
Family Tree DNA on Facebook
Ysearch (for those who have tested with another company and want to compare their results against the Family Tree DNA database)