What is a Y DNA Test?

Written by Ellen Hinkley
22 November 2016

Tracing your ancestry is one of the most common reasons for buying a home DNA test, boosted by the popularity of TV shows such as ‘Who Do You Think You Are?’ and celebrities such as Oprah Winfrey, who used a genealogy DNA test to find out about her African origins.

People are often surprised by the number of options there are when they start looking for an ancestry test to purchase. The three main types of test are autosomal, maternal and paternal – it’s the paternal tests that analyse your Y chromosome and are therefore known as Y DNA tests.

Your Y chromosome (Y DNA) is passed down to you (if you’re male) from your father’s father’s father etc. and is part of your genetic heritage. This means that it can be used to explore your paternal lineage. If you’re female, you can still trace your paternal ancestry by asking your biological brother, father, paternal uncle or paternal grandfather to take a Y DNA test. Y DNA tests are known to be most effective for tracing your paternal lineage up to 25 generations ago.

If you’d like to learn more about the types of Y DNA test that are on offer, you can visit our genetic ancestry listings, click the ‘Compare Tests’ button next to the genetic ancestry testing companies we’ve listed, and check out the ‘Paternal Analysis’ or ‘Paternal Analysis & Family Finder’ tests.

What is the Y chromosome?

Before taking a Y DNA test, it’s probably helpful for you to know a little bit more about the Y chromosome. As discussed, only males possess a Y chromosome and this is what determines gender. The Y chromosome contains about 200 genes (of the 20,000 to 30,000 in your whole genome), and only 50 to 60 of these 200 genes code for proteins – research is still being undertaken to discover exactly what these genes are responsible for.

Because mutations occur very infrequently in the Y chromosome, the relatively small amount of genetic variation can be used to trace what’s known as ‘deep’ paternal ancestry. Y DNA tests will help you understand the migratory paths of your paternal ancestors after they left Africa around 200,000 years ago, up until the thousand year period before the era of mass migration circa 1850. There are two main types of Y DNA test: STR and SNP.

STR Y DNA Tests

STR stands for ‘short tandem repeat’, and STR tests look for repeated sections in Y DNA. STRs arise naturally in Y DNA and these ‘mutations’ (or markers) can be used to trace ancestry. Every male possesses STRs in their Y chromosome, and the number of times that these sections are repeated can be used to differentiate between groups of male individuals. If only one STR was to be tested for, then 5-20% of males would possess it, making it difficult to differentiate between them. However, the more STRs tested for the lower the percentage of males that will possess that combination – this allows companies to establish groups that you’re more or less closely related to.

Although STR tests are traditionally used for relationship testing such as paternity testing, they can also be used to trace ancestry, with the degree to which two people share STRs indicating how closely they are related (if at all). An STR Y DNA test can therefore be useful for tracing your paternal ancestry by identifying individuals with whom you share your recent paternal lineage, or who shared your paternal lineage up until a point when mutations in the Y chromosome caused the lines to branch.

The companies offering these tests tend to include a ‘Family Finder’ feature in their results. This means that once your results have been processed, you’ll be able to see a list of the ‘living relatives’ who share your paternal lineage (aka ‘matches’ or ‘connections’) who you’ll be able to contact. For this reason, it can be useful to take a Y DNA test if you’ve hit a roadblock with traditional genealogy research.

SNP Y DNA Tests

The second type of Y DNA test is a SNP (single nucleotide polymorphism) Y DNA test. SNPs arise naturally in Y DNA and these are also a type of ‘mutation’ (or marker) that can be used to trace ancestry. SNPs are also known as ‘genetic variants’ and are analysed as part of other types of DNA test (e.g. genetic predisposition tests for your risk of prostate cancer). A SNP analysis doesn’t look at repeating sections but instead looks at the replacement, insertion or deletion of single bases in the Y chromosome (the Y chromosome is made up of 58 million base pairs: 116 million bases).

As with STRs, the degree to which two people share SNPs indicates how closely they are related (if at all), so this is another method of connecting with those who share your paternal lineage, providing the company you test with offers a Family Finder feature.

Only a few providers offer Y DNA tests that analyse SNPs (e.g. Full Genomes) but it’s worth noting that Family Tree DNA offer a SNP Y DNA test known as the ‘Big Y’, providing you’ve already bought a Family Tree DNA STR Y DNA test.

Haplogroups

Both STR and SNP Y DNA tests can help you identify your ‘paternal haplogroup’ (e.g. R1b), which is a code describing the group of people with whom you share a recent common male ancestor on your paternal line.

Paternal haplogroups have been established by analysing Y chromosome mutations in living population groups and archaeological remains. This has allowed experts to infer how groups of our paternal ancestors have diverged and migrated since leaving Africa approximately 200,000 years ago, and even back to the time of ‘Y-Chromosomal Adam’ (the oldest known male ancestor from which all humans are descended).

This research has been used to develop ‘phylogenetic trees’ for the Y chromosome (also known as Y haplotrees) which visually represent all paternal haplogroups and subgroups. Each branch represents a mutation(s) in the Y chromosome where a divergent population was created. Using various statistical techniques, the geographical location of the divergence can be estimated along with the date to within tens of thousands of years.

This means that by taking a Y DNA test, not only will you receive your paternal haplogroup, but the company providing it can give you the migratory path of your paternal ancestors over time. Many companies do this by providing colourful and interactive maps with descriptions of your paternal ancestors’ migratory journey.

You can also conduct your own research by identifying your paternal haplogroup on a phylogenetic tree, and following the branches to discover other groups to whom you’re related (albeit less closely than your own group). There are many online resources and ancestry forums to help you do this should you wish.

The difference between those in your haplogroup and your relatives

Although paternal haplogroups can be used to identify those with whom you share your paternal lineage, this doesn’t mean that you’ll be closely related to them. Family Tree DNA currently offer Y DNA tests that come with a Family Finder feature, which allow you to identify those in your paternal haplogroup (or a closely linked paternal haplogroup), providing they’ve also taken a Y DNA test with Family Tree DNA. Many of these living relatives will be 4th cousins or more distant. Technically speaking, you’ll share a male ancestor with these living relatives, but it is rare for a provider to hold any details about that shared ancestor – only in the case of a 3rd cousin or closer might you discover when or where that shared ancestor lived.

Do all Y DNA testing companies offer a Family Finder feature?

Although many companies offer a Family Finder feature, only Family Tree DNA holds ‘matching databases’ for Y DNA, mtDNA and autosomal DNA. This means that if you’re hoping to find living relatives on your paternal line, you’ll only be able to do this with a Family Tree DNA Y DNA test.

All other companies with a Family Finder feature (e.g. AncestryDNA) only hold matching databases for autosomal DNA, even though some of these companies may test your Y DNA to produce your paternal haplogroup (e.g. 23andMe). In these cases, the living relatives they identify will be matched according to your autosomal DNA, not your Y DNA. This means you will share ancestors with these relatives, but you’ll be unable to tell if they share your paternal lineage.

Don’t forget, when it comes finding living relatives, companies with a Family Finder feature will only be able to compare like with like to identify matches, for example: Your Y-DNA results to their Y-DNA database, your mtDNA results to their mtDNA database, or your autosomal DNA results to their autosomal DNA database. If you want to maximise the number of living relatives you find using DNA testing, we recommend you test all three types of DNA, and that you take Y DNA and mtDNA tests with Family Tree DNA.

How many markers should you include in a Y DNA test?

The number of markers analysed for a Y DNA test can affect your results. Although you won’t be given a choice in terms of the number of markers that are analysed for SNP Y DNA tests, you will be given a choice for STR Y DNA tests – the number of STRs you can choose ranges from 12 to 111.

The more markers you choose, the higher the accuracy of your match to the living relatives identified (providing that company offers a Family Finder feature). Whilst most companies with a Family Finder will return a long list of relatives (sometimes in the hundreds), it’s important to remember that in many cases, matching will be based on a common ancestor as many as 15 generations ago (several hundreds of years). Therefore, to identify living relatives with whom you share a recent ancestor, tests with more markers will help you focus on the right individuals in what will be a relatively large group.

In terms of how many STR markers to opt for, 12 is enough if you’re only really interested in your paternal haplogroup, or if you want to confirm relationships with those whom share your surname who’ve also taken a STR Y DNA test (providing you both take it with the same company). However, if you want to find living relatives without your surname and you’d like information about when you last shared a common ancestor with them, you are better off buying a STR Y DNA test that analyses 37 markers or more.

What to expect from a Y DNA test kit

Y DNA tests can come in a package of tests that include mtDNA and autosomal DNA analyses, or they can be purchased alone. 23andMe sell a package of tests for £149 and this is a cost-effective option when you consider that Family Tree DNA charge £135 for their Y-DNA37 test, which, as the name suggests, only includes a Y DNA analysis.

In terms of what to expect when you receive your test kit, analysis of your DNA is based on a saliva sample, provided via a swab or saliva collection tube, which is quick and painless. You then send off your sample to the lab for analysis, and you’ll receive your results in an online account.

How is a Y DNA test different to an autosomal DNA test or a mitochondrial DNA test?

We inherit 46 chromosomes from our parents, two of which (X & Y) are sex chromosomes: Females have two X chromosomes and males have one X and one Y. The remaining 44 are known as autosomes or autosomal DNA. Autosomal DNA testing can determine the varying ethnicities that your DNA is associated to, although it’s not possible to say which parent you inherited which ethnic segments from unless your parents take a test as well. An autosomal DNA test (aka an ethnicity test) is known to be most effective for investigating your ancestry from both parents within the past five generations.

We also inherit some DNA exclusively from our mothers and this is known as mitochondrial DNA (mtDNA). Mitochondrial DNA testing can be used to explore your maternal lineage as it’s passed down to you from your mother’s mother’s mother etc. mtDNA tests are known to be most effective for tracing your maternal lineage up to 52 generations ago.

Autosomal and maternal tests can also be used to help you find living relatives, providing the company you purchase the test from offers a Family Finder feature.