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Big Y-500 STR Matching

Roberta Estes

Family Tree DNA recently introduced Big Y-500 STR matching for men who have taken  the Big Y-500 test. This is in addition to the SNP results and matching. If you’d like an introduction or definition of the terms STR and SNP, you can read about SNPs and STRs here.

Beginning in April 2018, Family Tree DNA included an additional 379+ STR markers for free for Big Y testers as a bonus, meaning for free, including all earlier testers.

While the Big Y-500 STR marker values have been included in customers’ results for several months, unless you contacted your matches directly, you didn’t know how many of those additional markers above 111 you matched on – until now.

If you haven’t taken the Big Y test, the article Why the Big Y Test? will explain why you might want to. In addition to the Big Y results, which refine your haplogroup and scan the entire gold standard region of the Y chromosome looking for SNPs, you’ll also receive at least 389 Y STR markers above the 111 STR panel for total of at least 500, for free – which is why the name of the Big Y test was changed to the Big Y-500. If you haven’t tested at the 111 marker level, don’t worry about that because the cost of the upgrade is bundled in the price of the Big Y-500 test. Click here to sign in to your account and then click on the blue upgrade button to view pricing.

Big Y-500 STR Matching

To view your matches and values above the traditional 111 makers, sign on to your account and click on Y DNA matches.

You’ll see the following display.

The column “Big Y-500 STR Differences” is new. If you have not taken the Big Y-500 test, you won’t see this column.

If you have taken the Big Y-500, you’ll see results for any other man that you match who has taken the Big Y-500 test. In this example, 5 of this person’s matches have also taken the Big Y-500 test.

What Are Big Y-500 STR Differences?

The “Big Y-500 STR Differences” column values are expressed in the format “4 of 441” or something similar.

The first number represents the number of non-matching locations you have above 111 markers – in this case, 4. In the csv download file, this value is displayed in the “Big Y-500 Differences” column.

The second number represents the total number of markers above 111 that have a value for both of you – in this case, 441. In other words, you and the other man are being compared on 441 marker locations. In the csv download file, this value is displayed in the “Big Y-500 Compared” column.

Because the markers above 111 are processed using NGS (next generation sequencing) scan technology, virtually every kit will have some marker locations that have no-calls, meaning the test doesn’t read reliably at that location in spite of being scanned several times.

It’s more difficult to read STRs accurately using NGS scan technology, as compared to SNPs. SNPs are only one position in length, so only one position needs to be read correctly. STRs are repeated of a sequence of nucleotides. A 20 repeat sequence could consist of 20 copies of a series of 4 nucleotides, so a total of 80 positions in a row would need to be successfully read several times.

Let’s take a look at how matching works.

How Does Big Y-500 STR Matching Work?

If you have a total of 441 markers that read reliably, but your match has a total of 439 that produced results, the maximum number of markers possible to share would be 439. If you both have no calls on different marker locations, you would match on fewer than 439 locations. Here’s an example just using 9 fictitious markers.

Based on the example above, we can see that the red cells can’t match because they experienced no-calls, and the yellow cells do have results, but don’t match.

New Filter

There’s also a new filter option so you can view only matches that have taken the Big Y-500 test.

Let’s look at some of the questions people have been asking.

Frequently Asked Questions

Question 1: Are the markers above 111 taken into account in the Genetic Distance column?

Answer: No, the values calculated in the genetic distance column are the number of mismatches for the marker level you are viewing using a combination of the step-wise and infinite alleles mutation models. (Stay with me here.)

In our example, we’re viewing the 111 marker level, so the genetic distance tells you the number of mismatches at 111 markers. If we were viewing the 67 marker level, then the genetic distance would be for 67 markers.

The number of mismatches above 111 markers shows separately in the “Big Y-500 STR Differences” column and is calculated using the infinite alleles model, meaning every mutation is counted as one difference. You can read more about genetic distance in the article, Concepts – Genetic Distance.

The good news is that you don’t need to calculate anything, but you may want to understand how the markers are scored and how the genetic distance is calculated. If so, go ahead and read question 2. If not, skip to question 3.

Question 2: What’s the difference between the step-wise model and the infinite alleles model?

Answer: The step-wise model assumes that a mutated value on a particular marker of multiple steps, meaning a difference between a 28 for one man and a 30 for another is a result of two separate mutation events that happened at different times, so counted as 2 mutations, 2 steps, so a genetic distance of 2.

However, this doesn’t work well with palindromic markers, explained here, where multi-copy markers, such as DYS464, often mutate more than one step at a time.

Counting multiple mathematical differences as only one mutation event is called the infinite alleles model. For example, a dual copy marker that has a value of 15-16 could mutate to 15-18 in one step and would be counted as one mutation event, and one difference and a genetic distance of one using the infinite alleles model. The same event would count as 2 mutation events (steps) and a genetic distance of 2 using the step-wise mutation model. In this article, I explain which markers are calculated using which methodology.

Another good infinite alleles example is when a location loses it’s DNA at a marker entirely. If the marker value for most men being compared is 10 and is being compared to a  person with no DNA at that location, resulting in a null value of 0 (which is not the same as a no-call which means the location couldn’t be read successfully), the mutation event happened in one step, and the difference should be counted as one event, one step and a genetic distance of one, not 10 events, 10 steps and a genetic distance of 10.

To recap, the values of markers 1-111 are calculated by a combination of the step-wise model and the infinite alleles model, depending on the marker number and situation. The differences in markers above 111 are calculated using the infinite alleles model where every mutation or difference equals a distance of one unless a zero (null) is encountered. In that case, the mutation event is considered a one. However, above 111 markers, using NGS technology, most instances where no DNA is encountered results in a no-read, not a null value.

Question 3: Has the TIP calculator been updated?

Answer: No, the TIP calculator does not take into account the new markers above 111. The TIP calculator relies upon the combined statistical mutation frequency for each marker and includes haplogroup differences. Therefore, it would be difficult to compensate for different numbers of markers, with various markers missing for each individual above 111 markers. The TIP calculator only utilizes markers 1-111.

Question 4: Do projects display more than 111 markers?

Answer: No, projects don’t display the additional markers, at least not yet. The 111 marker results require scrolling to the right significantly, and 500 markers would require 5 times as much scrolling to compare values. Anyone with an idea how to better accomplish a public project display/comparison should submit their idea to Family Tree DNA.

Question 5: Which markers above 111 are fast versus slow mutating?

Answer: Results for these markers are new and statistical compilations aren’t yet available. However, initial results for surname projects in which several men who share a surname and match have tested indicate that there’s not as much variation in these additional markers as we’ve seen in the previous 111 markers, meaning Family Tree DNA already selected the most informative genealogical markers initially. This suggests that the additional markers may provide additional mutations but probably not five times as many as the initial 111 markers.

Question 6: Why do I have more mutations in the first 111 markers than I do in the 389+ markers above the 111 panel?

Answer: That’s a really good question. You’ve probably noticed in our example that the men have dis-proportionally more mutations in the first 111 markers than in the markers above 111.

The trend is clearly for the first 111 markers to mutate more frequently than the 379+ markers above 111. This means that the first 111 markers are generally going to be more genealogically informative than the balance of the 379+ markers. However, and this is a big however, if the line marker mutation that you need to sort out your group of men occurs in the markers above 111, the number of mutations and the percentages don’t mean anything at all. The information that matters is how you can utilize these markers to differentiate men within the line you are working with, and what story those markers tell.

Of course, the markers above 111 are free as part of the Big Y-500 test which is designed to extract as much SNP information as possible. In essence, these STR markers are icing on the cake – a treat we never expected.

Bottom Line

Here’s the bottom line about the Big-Y 500 STR markers. You don’t know what you don’t know and these 379+ STR markers come along with the Big Y test as a bonus. If you’re looking for line-marker STR mutations in groups of men, the Big Y-500 is a logical next step after 111 marker testing.

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