What is a Heteroplasmy and Why Do I Care?

Most people have never heard of a heteroplasmy – but you might have one.

You Might Have a Heteroplasmy If…

…You have no exact matches at the full sequence mitochondrial DNA level.

A heteroplasmy is one of the first things I think of when someone tells me they have no exact full sequence matches but several that are a genetic distance of 1, meaning one mutation difference.

That phenomenon usually means the tester has a rare mutation that no one else has, at least no one who has tested their mitochondrial DNA (yet) – and that mutation just might be a heteroplasmy.

Heteroplasmies are generally (but not always) quite recent mutations. Actually, heteroplasmies are mutations caught in the act of mutating – kind of like an insect in genetic amber – frozen in time in your generation.

By Anders L. Damgaard – http://www.amber-inclusions.dk – Baltic-amber-beetle CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16792582

Let’s say you might have a heteroplasmy. Or maybe you want to see if you do. Even if YOU don’t have a heteroplasmy, other people’s heteroplasmies can and will affect matching.

Here’s everything you ever wanted to know about heteroplasmies but didn’t know to ask😊

Heteroplasmies are Fascinating

A heteroplasmy is actually quite interesting because it’s a genetic mutation in progress.

This means you have two versions of a DNA sequence showing in your mitochondrial DNA at a specific location.

Said another way, at a specific genetic location, you show both of two separate nucleotides. Amounts detected of a second nucleotide greater than 20% are considered a heteroplasmy. Amounts below 20% are ignored. Generally, within a few generations, the mutation will resolve in one direction or the other – although some heteroplasmies persist for several generations and can sometimes define family branches.

If you’d like to read more about mitochondrial DNA, I wrote a series of step-by-step articles and combined them into one resource page, here.

Show Me!

You can easily check to see if you have a heteroplasmy by signing on to your FamilyTreeDNA account. Hopefully, you’ve taken the full sequence test.

Today, new testers, thankfully, can only purchase full sequence tests, so HVR1 results don’t present quite the same challenges when combined with heteroplasmies as they used to. We’ll talk about that in a minute.

If you have only taken the HVR1 or HVR1+HVR2 “Plus” test, as opposed to the Full Sequence, you can upgrade by signing on here and clicking on the “Full” button on the Maternal Ancestry section of your personal page.

These buttons will be pink if you’ve taken that test already, and grey if you need to upgrade. If you have an account at FamilyTreeDNA, you can add a mitochondrial DNA test to that same account by clicking on “Add Ons and Upgrades” at the top of your personal page. You can order a test if you’re a new customer, here.

How Do I Know if I Have a Heteroplasmy?

Your mitochondrial DNA has a total of 16,569 locations that you can think of as addresses. If your DNA at those locations is normal, meaning no mutations, they won’t be listed in your results.

Mutations are shown in your mitochondrial DNA results by a different letter at the end of the location.

For example, here are my mutations for my HVR1 region. Each of these locations in the HVR1 region has a mutation.

For locations that are shown in your results, meaning those where you have a mutation, you’ll see, in order:

  • A letter, either T, A, C or G
  • The location number
  • A different letter, typically another one of T, A, C or G, but sometimes a small d

For the first mutation, C16069T, the location address is 16069, the normal value is C, the mutation that occurred is T.

Heteroplasmies are shown in your mitochondrial DNA results by letters other than T, A, C, G or d at the end of the location.

I don’t have any heteroplasmies, so I’m switching to the results of a cousin who has a heteroplasmic mutation at location T16362Y to use as an example. The trailing Y means they have a heteroplasmy at location 16362.

But first, what do those letters mean?

The Letters

The letters stand for the nucleotide bases that comprise DNA, as follows:

  • T – Thymine
  • A – Adenine
  • C – Cytosine
  • G – Guanine
  • d – a deletion has occurred. There is no nucleotide at this location.

For location T16362Y, the first letter, T, is the “normal” value found at this location. If a mutation has occurred, the second letter is the mutated value. Normally, this is one of the other nucleotides, A, C or G.

Any other letter after the location has a specific meaning; in this case, Y means that both a C and a T were found, per the chart below.

Note – if you have a small letter t, a, c or g, it’s not a heteroplasmy, and I wrote about small letters and what they mean in the article, Mitochondrial DNA Part 2: What Do Those Numbers Mean?

Check Your Results

On your FamilyTreeDNA personal page in the mtDNA section, click on the Mutations tab.

If you’ve taken the full sequence test, you’ll see Extra Mutations. You’re looking for any mutation that ends in any letter other than T, A, C, G or d.

If you haven’t taken the full sequence test, you don’t have “Extra” mutations listed, but you can still view your mutations for the HVR1 and HVR2 regions.

Look for any value that has any letter other than T, A, C, G or lower case d at the end of the location.

The Y tells us that this location is a heteroplasmy.

Heteroplasmy Matching

Ok, let’s look at a heteroplasmy mutation at location 16326. A heteroplasmy can occur at any mitochondrial location. I’ve selected this location because it occurs in the HVR1 region of the mitochondrial DNA, so even people who haven’t tested at the full sequence level will see results for this location. Plus, the location at which the heteroplasmy occurs affects matching in different ways.

Using the example of T16362Y, the Y tells us that both nucleotides C and T were found. This location should match against anyone carrying the following values in the same location:

  • Y (letter indicating a C/T heteroplasmy)
  • T (standard or normal value)
  • C (mutated value)

However, currently at Family Tree DNA, the heteroplasmy only counts as a match to anyone with a Y, the specific heteroplasmy indicator, and the “normal” value of T, but not the mutated value of C.

This table shows how heteroplasmies are counted at FamilyTreeDNA. For heteroplasmy T16362Y, based on the value your potential match has at this location, you either will or will not be considered a match at that location.

Scenario Other Person’s Value Your Result – T16362Y
1 T16362Y – heteroplasmy indicator Match to you at this location
2 T16362T – normal value, not a mutation Match to you at this location
3 T16362C – mutated value Not counted as match to you at this location
  • If your match has a value of Y, the heteroplasmic C/T value, they are counted as a match to you, so no problem.
  • If your match has a value of T, the normal value, this location won’t be shown on their mutation list at all. They WILL be counted as a match to you so there’s no issue.
  • If your match has a value of C, the mutated value, in my opinion they should also be counted as a match to you, but they aren’t today. The logic, I believe, was that the most likely value is the standard or normal value and that the mutated value is much less likely to be accurate. Regardless, I’ve requested this change and am hoping for a matching adjustment in a future release for heteroplasmies.

Heteroplasmies do affect matching at the different levels.

Viewing Your Matches

Mitochondrial DNA, for testing purposes, is broken into three regions, HVR1 (hyper-variable region 1), HVR2 and the Coding Region.

At FamilyTreeDNA, you can view your matches at each level. The matches are cumulative, meaning that the HVR2 level includes the HVR1 level information, and the Coding Region level includes the HVR1 and HVR2 regions. That highest level which includes all three regions shows information from your entire your entire full mitochondrial DNA sequence.

Heteroplasmy Effects on Matching

If you otherwise match someone exactly, but one of you has a heteroplasmy and the other person carries the mutated value, you will be counted as a mismatch of 1 at the full sequence level.

A mismatch has different effects when it occurs in the HVR1, HVR2 or Coding Regions, respectively.

GD is an abbreviation for Genetic Distance which is how mutations are counted. A GD of 1 means the two people have one mutation difference between them.

In the following chart, the effects of you having a nonmatch, heteroplasmic or otherwise, in each of the regions is shown at each level. The region in which the mismatch occurs is shown in the first column, at left, and the effect the mismatch has on matching in each region is shown in columns 2-4.

The red sections are not counted as matches.

Mismatch Occurs in this Region HVR1 Level Match to Someone Else HVR2 Level Match to Someone Else Coding Region Level Match to Someone Else
HVR1 region nonmatch GD of 1 means no match GD of 1 means no match GD of 1 is a match
HVR2 region nonmatch Does not affect HVR1 – so you are a match GD of 1 means no match GD of 1 is a match
Coding Region nonmatch Does not affect HVR1 – so you are a match Does not affect HVR2 – so you are a match GD of 1 is a match

For purposes of this discussion, we’re assuming our two people being compared in the chart above match exactly on every other location so matching is not otherwise affected.

  • If your heteroplasmic nonmatch occurs in the HVR1 region – in other words, scenario 3 – you’ll fall into the HVR1 nonmatch row. That means you won’t be shown as a match at the HVR1 or HVR1+HVR2 levels, but you WILL be shown as a full sequence match.
  • If your heteroplasmic nonmatch is in the HVR2 region of addresses, it won’t affect your HVR1 matches, but it will affect your HVR2 and Coding Region matches. This means you will be shown as HVR1 match, not an HVR2 match, but will be a full sequence match.
  • If your heteroplasmic nonmatch is in the Coding Region, it won’t affect your HVR1 or HVR2 matches, but it will affect your Coding Region matches. However, it won’t preclude matches and you’ll be shown as a match in all three regions.

To be very clear, I have no issue with these match thresholds. It’s important to understand how this works, and therefore why heteroplasmic (and other) mismatches in specific regions affect our matches in the way they do.

Why Aren’t Mismatches of 1 Counted as Matches in the HVR1 or HVR2 Regions?

The match threshold at FamilyTreeDNA for the HVR1 and the HVR1+HVR2 regions, both small regions of about 1000 locations each, is that only an exact match is considered a match. Therefore, a heteroplasmic nonmatch in this region can really be confusing and sometimes misleading, especially if either or BOTH people have NOT tested at the full sequence level.

These are the match thresholds in effect today.

HVR1 GD or # of Mutations Allowed for a Match HVR2 GD or # of Mutations Allowed for a Match Coding Region GD or # of Mutations Allowed for a Match
0 – no mutations allowed 0 – no mutations allowed 3 mutations allowed

If both people match on either the heteroplasmy identified (Y in our case) or one person has the normal value – all is fine. But if one person has a heteroplasmy and the other has the mutated value – then a mismatch occurs. This is really only problematic when:

  • The heteroplasmy mismatch is in the HVR1 region and both people have only tested at that level, causing the two people to not match at all.
  • The heteroplasmy mismatch occurs in combination with other mutations that, cumulatively, push the two people over the GD 3 full sequence matching threshold.

The second scenario happens rarely, but I have seen situations where people don’t match their mothers, aunts, siblings, or other close relatives because of multiple heteroplasmic mutations occurring in different people.

And yes, this is hen’s teeth rare – but it does occasionally happen.

So, what’s the bottom line about heteroplasmies?

Heteroplasmy Bottom Line

  1. You can suspect a heteroplasmy if you have full sequence matches, but no exact matches.
  2. If you have a heteroplasmy in the HVR1 region, understand that you may not have many or any matches in the HVR1 and HVR2 regions. The remedy is to test at the full sequence level and check matches there.
  3. If you have a heteroplasmy and don’t match someone you expect to match – reach out to them and ask about their value at that specific location. If that location isn’t listed for them in their results, then they have no mutation there and your heteroplasmy is NOT the cause of you not matching with them.
  4. If you don’t match someone you expect to match, reach out to them and ask if THEY have any heteroplasmies. The easiest way to ask is, “Do you have any mutations listed that end with anything other than T, A, C, G or d?” Feel free to link to this article so that they’ll know where to look, and why you’re asking.

Do you have any heteroplasmies?

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