The first Y-chromosome DNA (Y-DNA) tests came onto the market nearly 20 years ago, but the focus in recent years has been on autosomal DNA (atDNA) testing – the type of test offered by AncestryDNA and other companies that gives you matches with genetic cousins who could be related to you on any of your ancestral lines.
Y-DNA testing has taken a back seat, but it is still a very important tool for the family historian.
While atDNA is useful for answering questions about relatedness within the last five or six generations, it cannot be reliably used for matches with distant cousins because of the way it is shuffled up and diluted with each new generation.
The Y-chromosome, in contrast, is passed on virtually unchanged from father to son.
It is a useful tool for answering questions about relatedness on the direct male line (the patriline) going back to the beginning of genealogical records in medieval times.
The big limitation is that only males have a Y-chromosome, so the test can only be taken by men.
If you’re a female you’ll need to find a male relative to take a Y-DNA test on your behalf.
As with all DNA tests, it is important to combine Y-DNA results with genealogical research.
A DNA test on its own provides very little useful information, and the power of the test lies in the ability to compare results in a matching database.
FamilyTreeDNA (FTDNA) dominates the market for Y-DNA testing, and is now the only company that has a Y-DNA matching database.
If you’ve tested with 23andMe or Living DNA then you will receive information about your Y-DNA haplogroup (your branch on the Y-DNA tree).
While these reports can be very interesting, they relate to your deep ancestry thousands of years ago, and cannot be used for genealogical matching.
How to use Y-DNA testing
In most cultures Y-DNA tracks the same line of inheritance as surnames.
A Y-DNA test can be used to answer questions such as whether two men with the same surname from different parts of the country share a common ancestor, or whether two variant spellings of a surname have a common root.
You will get the most out of a Y-DNA test if there is already a structured project for your surname.
If you’re the first person with your surname to test then you are not likely to get much from your results, but if there is a well-established project you are more likely to have a useful match.
Even if you don’t, your results will still contribute to the knowledge of the surname.
Y-DNA testing can also be used in unknown-parentage searches.
For example, if someone was adopted or donor-conceived, or if the patrilineal ancestor was illegitimate and the father’s name was not given on the birth certificate.
In these situations you are relying on the power of the matching database, and you are hoping for close matches that will provide clues to the biological surname of the father.
Some people get answers straight away, while in other cases people have to wait for many years for the right match to come along.
What is tested?
There are two different types of marker that are tested – short tandem repeats (STRs) and single-nucleotide polymorphisms (SNPs).
STRs are short repeating sequences of DNA letters.
The number of repeats is counted up, and you are assigned a number for each position or marker that is tested.
For example, if the motif TAGA is repeated 10 times, then you will be given the number 10 for this marker.
Your Y-DNA test result is presented as a string of numbers representing your genetic signature, which is known as a haplotype.
In simple terms, the more matching markers you share with another tester, the more likely you are to be related.
An SNP is a single change in a DNA sequence, for example a T in a father becomes a C in his son.
SNPs mutate very slowly, and are therefore very stable.
The man in whom the mutation first appeared passes it on to his sons, and all their descendants several thousand years later will also carry the same SNP.
Over time more SNPs accumulate, but the earlier SNPs are still preserved.
The Y-chromosome therefore contains a cumulative record of all of the SNPs that have ever occurred in a man’s paternal line.
When you get your Y-DNA results, the first thing you will want to do is look at your match list.
Matches are reported at up to five different testing levels depending on which test you ordered: 12, 25, 37, 67 or 111 markers.
It is generally only matches at 37 markers or higher that are likely to be meaningful.
FTDNA reports the genetic distance, which tells you the number of mismatching markers.
At 37 markers you are considered to match another person if you have a genetic distance of 0, 1, 2, 3 or 4.
If the genetic distance is 0 then you and your match will have identical results.
If you have a genetic distance of 4 then you will match on 33/37 markers.
Y-DNA is passed down from father to son Credit: Hulton-Deutsch Collection/Corbis via Getty Images
FTDNA provide some useful guidelines for interpreting your matches in its Learning Center.
The company estimates that if you have an exact match with a genetic distance of 0, then 95 per cent of the time such a match will fall within the last seven generations.
But at a genetic distance of 2 the match will be within the last 14 generations 95 per cent of the time.
Matches with a bigger genetic distance will span a much broader number of generations.
Many people find that they get matches with a variety of different surnames as the genetic distance increases.
What you are hoping for is to have very close matches with people with the same surname or a related variant spelling.
Some people have no matches at any testing level, while there are some men who have 1,000 or more matches at 37 markers with a variety of surnames.
If you don’t have any useful matches you can either wait for other people to test, or you can encourage a distantly related cousin, such as a fourth or fifth cousin, to take a test for comparison purposes.
If you have large numbers of matches at 37 markers, it will be necessary to upgrade to refine your matches, but this is only worthwhile if your matches have also upgraded to a higher level, so you have people you can compare your results with.
Y-STR tests are useful for answering questions about patrilineal relatedness in the last few hundred years.
However, in some circumstances it will not be possible to interpret the results with confidence, and SNP testing will be required.
This is particularly the case if you are trying to make connections back in the medieval era, when fewer genealogical records are available and the interpretation of Y-STR matches becomes more difficult.
Additional testing is also useful if you are interested in your deep ancestry and wish to refine your placement on the Y-DNA haplotree.
It is possible to order SNP packs that contain a sample of known SNPs for specific haplogroups, but a Big Y test or equivalent is likely to be a more viable and cost-effective option in the long run.
Y-DNA testing has come a long way, and we are likely to see more advances and falling prices.
Even if you don’t have good matches from the outset, your DNA will stay in the database as projects grow, and will serve as a legacy for future generations.