Concepts – Sibling and Twin DNA Matching

Lots of people are giving their siblings DNA test kits.  That’s a great idea, especially if your parents aren’t available for testing, because siblings do inherit part of the same DNA from their parents, but not all of the same DNA. That means testing siblings is a great opportunity for more genealogical matches!

Recently, a friend asked me why his fraternal twin has matches to people he doesn’t, and vice versa.  Great question, so let’s take a look at what to expect from matches with siblings.

First, identical twins share exactly the same DNA because they are created as a result of the division of the same egg that has been fertilized by the father’s sperm. Identical twins matches should be identical.

A fraternal twin is exactly the same as a sibling. Two separate sperm fertilize two separate eggs and they gestate together, at the same time.

Second, let’s talk just a minute about Y and mitochondrial DNA, then we’ll discuss autosomal DNA.

Full Siblings Share
Mitochondrial DNA Exactly the same, unless a mutation occurred
Y DNA Males will share exactly the same, unless a mutation occurred.  Females don’t have a Y chromosome.
Autosomal DNA Approximately 50% of autosomal DNA

To obtain detailed Y and mitochondrial DNA results, you’ll need to test with Family Tree DNA. They are the only vendor offering these tests.

For autosomal matching, you can test with a number of vendors including: Family Tree DNA, Ancestry, 23andMe and MyHeritage.

You can read more about the different kinds of testing here, and a comparison of the different tests and vendors here.

50% the Same – 50% Different

Siblings share approximately 50% of the same DNA of the parents.  The other 50% is different DNA that they received from the parents that the other sibling did not receive.

In the conceptual example above, you can see that each child inherited 4 segments of the 8 total offered by their parents.  Only two of those segments were the same for both siblings, segments 3 and 4.  Of these two siblings, no one inherited parental segments 7 and 8.  Perhaps a third child would.

In other words, siblings can expect to see many of the same people in their match list and several that are different. In our example, the same people would be matching both siblings on segments 3 and 4.  People matching child 1 but not child 2 would be matching on segments 1 and 2.  People matching child 2 but not child 1 would be matching on segments 5 and 6.

The reason you’ll see the same people on your match list is because you did inherit 50% of the same DNA from your parents.

There are two reasons you’ll see different matches on your match lists.

Some of your matches on your list that don’t match your sibling will be because the two siblings inherited different pieces of DNA from their parents.  Your sibling will match people on the DNA that they received from your parents that you didn’t receive, and vice versa.

Some Matches are Identical By Chance (IBC)

Another reason for different matches is because you and your sibling will have people on both of your match lists that don’t match either parent as a result of IBC or identical by chance matching. That’s where the DNA of your match just happens to match you by virtue of zigzagging back and forth between your Mom’s and Dad’s DNA that you carry.

As you can see in this example, your pink DNA came from your Mom, and blue from your Dad, but your match carries some of both values, T and A.  This means they match you, but not because they match either of your parents.  Just an accident of circumstance. That’s what IBC is.

Telling the Difference

I wrote about matches that are identical by descent (IBD), meaning because you inherited that DNA from your parents, and identical by chance (IBC) in this article.

Unfortunately, your DNA is mixed together and without other known relatives testing, it’s impossible to discern which DNA is inherited from your mother and which from your father. This is exactly why we encourage people to have known relatives test such as parents, grandparents and cousins.  Who you match on which segments indicates where those segments descended from in your family tree.

If one or both parents are living, that’s the best way of discerning which matches are identical by descent and which are by chance.

A recent project with Philip Gammon showed by segment size the likelihood is of a match being genuine or identical by chance.  If both parents have tested, he offers the free Match-Maker-Breaker tool to do this analysis for you.

The bottom line is that when comparing your matches to those of your siblings, about 20-25% of everyone’s total matches are identical by chance, especially those at lower centiMorgan levels.

The remaining 80% or so will be divided roughly half and half, meaning half will match you and a sibling both, and half will only match you. Therefore, you will be looking at roughly 40% of your matches being in common with a particular sibling, 40% not matching your sibling but being legitimate matches and the remaining 20% that are identical by chance.

Test Parents and Family Members

Of course, because you do share roughly half of the same DNA inherited from your parents, you will have some matches to both you and a sibling that are identical by chance in exactly the same way.  Just finding someone on both of your match lists doesn’t guarantee that the match ISN’T identical by chance.

The best way to eliminate identical by chance matching, of course, is to test your parents.  Sadly, that isn’t always possible.

The next best way to determine legitimate matches is to test other family members.  At Family Tree DNA, they provide customers with the ability to link the DNA tests of family members to their proper location in your tree, and then Family Tree DNA utilizes the common DNA segments to determine common matching between you, that family member(s), and other people.

Those people who match you and a family member on the same segment are then identified as either paternal or maternal matches, based on their position in your tree.

Identifying Lineage

When thinking about who to test, half-siblings, if you have any are, a wonderful way to differentiate between maternal and paternal matches.  Because you and a half sibling share only one parent – which side of your tree those common matches come from is immediately evident!

Of my matches at Family Tree DNA, you can see that of my total 3165 matches, 713 are paternal and 545 are maternal, with 4 being related to both sides.  Don’t get too excited about those “both sides” matches, they are my descendants!

Paternal and maternal bucketing is a great start in terms of identifying which matches are genealogical – and that’s before I do any actual genealogy work.  All I did was test, create or upload a tree and connect tested family members to that tree.

Family Tree DNA is the only vendor to offer this feature.

Ethnicity

Ethnicity is a slippery fish.  I generally only consider ethnicity estimates reliable at the continental level.  There are lots of reasons that siblings will receive somewhat different ethnicity results including the internal algorithms of the various vendors.  You can read about what is involved in ethnicity testing here.

Transfers Give You More For Your Money

If you test at one of the vendors, you may be able to transfer to other vendors as well as GedMatch.  In the chart below, you can see which vendors accept transfers from other vendors. You can read more here.

Have Fun

Lots of people are now testing their DNA and I hope you and your siblings will find some great matches among the new testers. The great thing about siblings, aside from the fact that they are your siblings, is that you can leverage each other’s DNA matches.  Just one more way to share and move the genealogy ball forward.

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18 thoughts on “Concepts – Sibling and Twin DNA Matching

  1. 12/28/2017: I am a member of 23 & Me. What is different, or what additional facts will I learn from the DNA test, compared to what I learned from 23 & ME??

  2. Thanks,very interesting.
    I don’t have any siblings,but I am concerned about mutations that could affect the Y DNA.
    My father’s paternal grandparents were 1st cousins.If there were a mutation because.of this,could my Y DNA be affected weakened or be therefore inherited from a different great grand parent?I may be talking rubbish but what sort of effects could a cousin marriage have on Y DNA?
    Or perhaps you could refer me to anything you have written on this topic in the past.
    Thanks,Peter Radford.

  3. Hi Roberta… can I buy FTDNA Family Finder tests for two siblings via your affiliate link? If so, would the test kits be mailed to me or could they be mailed directly to my sibs? Thanks for all the very interesting, informative and extremely well-written articles.

    • Yes, you can. When you purchase, you will have the opportunity to enter the name, address and e-mail of the kit. After you are assigned a kit number, which is immediately after you order (via a confirming e-mail), you can then at any time sign on to the kit and change anything you have entered. You can share the link with them too so they can do the same. You should make two separate purchases, one for each sibling.

  4. Hi Roberta. Great article! I was trying to explain this concept to my brother other Christmas. Very timely, so thank you! I do have 14 matches that also match to my mom’s cousin, but don’t match to my mom. Would these be identical by chance? Or could they match to my dad and also to mom’s cousin?

  5. I am a twin, my brother and I have different blood types. While he was in the military, his blood tested as b postive, while I was pregnant, my blood tested o negative. They retested 3 times! Is this common among faternal twins?

    • The fact that you are fraternal twins doesn’t make any difference. You would inherit as normal siblings. As for blood type inheritance possibilities, that’s a medical question and I’m not a doctor.

  6. Roberta, I really like the chart about DNA transfers. I run the blog for a small genealogical society in Monterey, CA and would like to get permission to share the chart on our site. Our organization name is Monterey County Genealogical Society. The site address is mocogenso.org.

    Thanks, Jim Robeson

  7. I found your website via searching for information on the CRUMLEY family. I am looking for information on Euphemia Crumley (1802-1881) who married Adam Hoover (1784-1854). I have been searching for months and cannot find her parents. Can you help?

    • No, but you might want to take the Family Finder test at Family Tree DNA, join the Crumley project and see if you match any of our participants. The link to Family Tree DNA is on the sidebar and the holiday sale is still in place through today.

  8. You helped me to understand why my sister and I have such different results. Thanks!
    I guess the same applies for children vs. parents? I expected my daughter’s ethnicity to be 50% of my ethnicity results, and 50% of her father’s. But it’s not. For example, I received a 10% ‘west Asian’ estimate, and she got 0%. To take an extreme example, does it mean that if my ethnicity is 50% Chinese and 50% Japanese, my daughter could in theory receive from me a full 50% Japanese and 0% Chinese, or 50% Chinese and 0% Japanese, or anywhere in between?

  9. My family finder and that of my brother are very different, though we are full brother and sister.
    Mine looks like it might be from my father, his looks like it might be from my mother. For example, he has a trace of less than 1% South American (I was told this indicates possibly Native American, our great great grandmother was Native American), I don’t have this. My trace result shows Central Asia less than 2%. How would you explain this?

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