AncestryDNA Is Using FIRs to Distinguish Full and Half Siblings

Did you know that AncestryDNA is using fully-identical regions to distinguish full siblings from half siblings?  And apparently has been doing so since 2016?  I didn’t.

If you’re asking yourself, ‘How did we miss that?‘, I am to!  And I don’t have a good answer.  I didn’t read their scientific White Paper closely enough.  Mea culpa.

Then again, if you’re asking ‘Isn’t a FIR a tree?  The other kind of tree.  What does it have to do with DNA?‘, read on.

A fir, not to be confused with an FIR (any excuse for a good botany pic!):

Autosomal DNA and Relationships

Autosomal DNA, the kind most commonly used in genealogy, indicates how closely two people are related.  The more DNA they share, the closer the connection is likely to be.

It’s not so simple, though, as you can see from the graph below (modified from Figure 5.2 in the AncestryDNA Matching White Paper).  Each relationship has a range of possible shared DNA—some 2nd cousins share more than average and some share less—and the ranges of different relationships overlap.

The lower the amount of shared DNA, the more overlap there is among relationships.  Barring endogamy or other factors, a match of 500 cM could only fit one of three relationship categories (the groups that include 1C, 1C1R, or 2C, represented by the red, navy, and yellow lines, respectively), whereas at 200 cM, there are at least four possible categories:  1C1R/navy, 2C/yellow, 2C1R/light green, 3C/black, and possibly even 3C1R/light blue.

Even the closest relationships can fall into overlapping zones.  A half sibling (dark green line) who shares an above-average amount could match on more DNA than a full sibling (medium blue) who shares less than average.  There is a way to tell full and half siblings apart, though.

 

Fully Identical Regions

We have two copies of each autosomal chromosome (the ones that don’t determine sex).  For each chromosome, half siblings can only match one another on one of their two copies, the one they inherited from their shared parent.  They won’t match on the other copy, which they each inherited from a different parent.

Full siblings share two parents, though, so they can match on both copies of each chromosome.  They will have some sections of DNA where they inherited the same segment from their mother and, in the same spot, where they inherited the same segment from their father.  This is called a “fully-identical region” (FIR), and half siblings won’t have them.  Full siblings will also have regions where they only match through one parent, and some where they don’t match at all.

Genetic genealogists have long known that we can use FIRs to distinguish full versus half siblings, but we are only able to visualize these regions at 23andMe and GEDmatch.  Here’s a visual comparison of three chromosome in my two kids at GEDmatch:

The solid green regions are where they inherited matching DNA from both me and their dad, the mostly yellow regions are where they match through just one of us, and the mostly red regions are where they inherited different DNA from each parent.

Half siblings will have red and yellow sections when viewed at GEDmatch but no solid green (FIRs), like this:

 

FIRs at AncestryDNA

At AncestryDNA, matches are categorized into groups based on how much DNA is shared.  For example, a match who shares between 650 and 1300 cM will be in the “First Cousin” category (although other relationships are possible), a match between 200 and 650 cM is in the “Second Cousin” category, and so on.

Full siblings normally fall into the “Immediate Family” group and half siblings into the “Close Family” one.  If those categories were delineated solely by a centimorgan threshold, full siblings who share less than 2200 cM would be classified as “Close Family” and half siblings who share more than that would fall into the “Immediate Family” category.

In the past, that’s exactly what happened.  But it seems that AncestryDNA updated their algorithm in 2016 to consider FIRs.  The weird thing is that no one noticed!  Or if they did, they didn’t make a fuss over it.  I certainly missed it.

This figure, also from the AncestryDNA Matching White Paper, shows how the consideration of FIRs (labeled IBD2 on the horizontal x axis) helps to distinguish full (orange) and half (red) siblings, and even to tell identical twins (purple) from a parent or child (blue).

 

Considering only total shared centimorgans (the vertical axis), a match of 2200 cM could be either a full sibling or a half sibling.  Both red and orange points fall there.  However, when you plot FIRs (IBD2) on the x axis, the two relationships are distinct.

Parent–child and identical twin relationships can now be similarly differentiated.  Of course, most people with those matches never doubted the relationship once age was considered.

This change is a big improvement for close relationships.  That it flew under the radar for so long is pretty amazing!

Take home lesson:  AncestryDNA is doing a better job of telling full sibs from half sibs than they once did.

 

78 thoughts on “AncestryDNA Is Using FIRs to Distinguish Full and Half Siblings”

  1. Thangs so much for the blog. What about in the case of a man fathering children with women who are sisters of each other (not related to the man!), or a mother having children with men who are brothers (not related to the woman)?. If the sibling parents were very similar genetically, it seems this algorithm for distinguishing full vs half sibling might not work.

    1. The cases you are describing are sometimes called “three-quarter siblings”, who will definitely share FIRs, but not as many as full siblings. I haven’t seen a plot for 3/4 siblings, but I suspect Ancestry would categorize them as “Immediate Family” along with full siblings. It would be great to be able to tell them apart.

      1. My brother appears to be my 3/4 sibling. We match at 2109 cM across 64 segments. Is this conclusive? Test was at AncestryDNA.

        1. When you click on the little “i” in a grey circle beside the amount of shared DNA, what does it say?

    2. I used Visual Phasing to help a woman determine if she and her 3 siblings were full or 3/4 siblings. FIR alone couldn’t tell, but comparing them in pairs in the Visual Phasing spreadsheet I could tell there were two closely related fathers.

  2. Huh. When I discovered in 2016 (at age 64) that I was adopted, the adoptee search community (and everyone else!) kept hammering the point that the only way to validate my matches was with a chromosome browser. Fine, being completely new to DNA for genealogy I took their advice. Having tested with Ancestry I also took their advice to transfer my results to GEDMatch, and to “fish in all of the ponds” so I also tested at FTDNA and 23andMe. I got nowhere fast because my closest matches were all on Ancestry and once you get a reply in Ancestry’s horrible messaging system (if you ever do), adoptees (aka total strangers) generally don’t have a lot of success with getting a match to transfer to GEDMatch.

    So, I just rolled with the flow and gave up on chromosome browsing. The clues I had already within my Ancestry matches seemed to validate themselves with the estimated relationships (in cM’s and degrees) and by validating their trees using Ancestry’s excellent record databases. I created spreadsheet matrices of my Shared Matches because I could clearly see at least two distinct Shared Match groups that indeed turned out to be my paternal and maternal lines. The matrices also showed other groupings that further split the two lines, and I was able to target specific branches by using those matches with public tree starting points for my record research.

    I was amazed at how well Ancestry’s much maligned Timber and Underdog (mostly Underdog) algorithms appeared to work for me and I shared my experience in multiple groups & forums. Not very many people listened.

    But two years later I knew who both of my birth parents are (Dad is deceased) and am enjoying conversations with my Mom and meeting my maternal and paternal half siblings, nieces & nephews, great nieces & nephews.

    Whatever it is that Ancestry does (now I know the final puzzle piece, so thank you!) has had me as a minority but solid believer since 2016. It is also why I’m skeptical that the “new” methods of match visualizations (colored Shared Match matrices) will result in similar insights with test results from companies other than Ancestry.

    I’d love to see you do an in depth article on the matching methods that My Heritage does since it’s the only other company I’m aware of that claims to do phasing.

    1. Congratulations on finding your birth parents, and especially on the positive reunion with your mother’s side. I agree with you about chromosome browsers: I rarely use them myself (only for really specialized scenarios), and I solve most of my unknown parentage cases with just Ancestry.

    2. Jim,
      Are you able to share more info on your spreadsheets? I want to continue to improve my methods. My husband was adopted and within 72 hours of getting his results I was able to confirm the identity of his birth mom and hot on the trail of his birth father. I did this with one unlinked tree of an unresponsive first cousin.
      I am an NPE myself and found the identity of my two father options (brothers) in a matter of hours with a 1C match who was unresponsive.
      I’ve just confirmed the father as being one of two brothers for a cousin in Australia who had no first cousin matches.
      I’m currently working on finding the father for another NPE.
      I’ve invested HOURS of time and research at no charge for people I know and the only way I can help more people is to become more efficient. I would love to know more about your sorting and matrices.
      My husband’s maternal family and us are instantly bonded and it’s an amazing dream come true. It keeps me going and searching. Congrats to you and thanks for any information you are able to share.

  3. Have recently been contacted by 1/2 a twin (male & female) born illegitimately like myself to whom we’ve been able to determine father’s identity via 1st cousin links. This party’s mother also confirmed father’s name when confronted. I am a bit confused as to how we link since we have different mother, albeit same father. Apparently I was contacted as 1st cousin but when comparing connection we determined we’re half siblings. Would the twins both carry similar genetic material to me (I’m the younger of them) or what?? I filled them in on what research I had done on my father’s line until my nephew and cousin plus 3rd cousins decided to hide the family tree from me. I wasn’t wanting connection but did want to know my biological father’s line for my tree. Was told same thing happened with them.

      1. I believe he said he was over the 1,000cM range. With me being female I am 868.8cM to which I was told of 1st cousin and gr. nephew that enabled me in determining my biological father. His sister is a bit lower than I and that was all the info he gave me. When he wrote he said I was cited 1st cousin to him but we were able to conclude the biological parent being our father.

        1. One thousand centimorgans is too low for a half sibling. Exactly how much DNA do you share with the male twin? How much do you share with the female twin?

  4. He didn’t volunteer much other than to say that I, and some parties mentioned in my match also matched he. (This included 3rd – 5th cousins but not a 2ndC1R as this party is on my mum’s side). He did say his cM count was higher and his sister’s lower to mine – I’m female and mine is 868.8 to my 1st cousin and gr. nephew. Within this match and 3rd – 5th cousin matches I was able to determine my biological father

  5. As I haven’t heard from party, as yet, I reread his email sent and it appears I wrote incorrectly. His count of over 1,000cM isn’t to me but the same party I have for 1st cousin and gr. nephew and likewise to my 3rd – 5th cousins; however, not all of the latter. My name was listed in amongst his ‘match’ listings on the second page within ‘high’ connections. His sister, other half, has a near 700cM and, I’m assuming, this means we are closer linked than I to her brother. Within his first set of matches/links, as said earlier, we have same majority of named parties (none of respective mothers although looking at his research there seems to be a link between us at gr. gr. uncle lineage – have to check more into this as may be coincidence of same names and spouses with circa years entered) all of whom are on my biological father’s lines. I was adopted whereas they were raised by their mother and step-father – I did reunite with my biological mother and family and was told name of father which appears confirmed by my DNA match of 1st cousin and great nephew – top of the listings.

    1. Sounds like you’re on the right path. If you’re interested in hiring me for a consultation to guide your next steps, please email me at theDNAgeek (a) gmail.com.

      1. Hate to ask this but for how much?? I am really trying to get a bead on my ancestry/heritage lines more for health reasons as it appears both sides of my lines don’t have long lives and am curious to know why. I’ve found cousins and aunts dying in their 50′, 60’s with only a few making it into 80 years. Earlier, like 1700 and early 1800 had better lifespans – I know many believe evolution brought many changes but feel there is more to it than that. New family I’ve reached out too aren’t interested in the past (citing they have enough to worry about with today/tomorrow) and/or would prefer no communication so am busy trying to determine my father’s side, in full, and acquire death certificates. I am in my 60’s and resources are limited, I’m afraid.

        1. I have a brother and sister who have tested. They share 2342 cM per Ancestry and are reported as full siblings. I suspect that they are 3/4 siblings sharing a mother with the fathers being brothers. So both uploaded to GEDmatch and the total cM of FIR’s is 408, which is much lower than the appx 850 that I was suspecting. Would this indicate 3/4 siblings? Thank you for your thoughts.

        2. It’s difficult to tell full from three-quarter siblings. Unfortunately, I do not have a set of distributions that could tell you for sure one way or the other.

  6. Interesting information. I have a question. Will an Autosomal DNA test confirm that me and my half sister are indeed sisters when we share the same Father? He acknowledges her and my half brother (her full brother) and our older half sister. But refuses to acknowledge me. Years ago my half brother was willing to take a DNA test but back then they were $400. Now he isn’t even speaking to his sister or mom do I doubt I could get him to take the test(or if it would even tell us anything). My sister just submitted her test but I am now worried it will not show any relation and that it will be a waste of $69.

    1. If she’s your half sister, you and she should match in the “Close Family” category at AncestryDNA and should share about 1800 centimorgans (could be more, could be less, but it’ll be in that range). One additional way to confirm that you share a father would be to both transfer your raw data files to GEDmatch and do a “One-to-One X-DNA Comparison”. Sisters who share a father should match across the entire X chromosome. (A little bit missing at the end is nothing to worry about … that’s just an artifact.)

      1. I forgot to share our results
        Shared DNA
        25.3% (1,793.1‎ cM)
        Shared segments
        36
        Largest segment
        121.7‎ cM

        We actually got the results two days before Father’s Day 2019

        1. Oh and it was at My Heritage and puts us in
          Niece, half sister relationship category.

  7. my sibling brother and I shared 2135 cm on family tree DNA and DNA Painter showed it to be about an 81.42% likelihood of 1/2 siblings – only 18.58% full sibling.
    I recently took an Ancestry test and this one says 2175 over 69 segments and says full sibling. I had a little reason to think we might be something like 3/4 because I had suspected we might have different fathers. But the fathers would have been related..as their grandfathers would have been brothers. In other words my brother “could” be about a 3rd cousin on the male side…any info or input would be greatly appreciated. We are older and parents are deceased.

    1. At Ancestry, click on the little “i” in a grey circle next to the amount of shared DNA. What does it say there about the percentage chance that you are full siblings or half siblings?

  8. I have had a dna test with my half sister, it tested 25 strands and was inconclusive. They then tested 35 strands and was the same result. Would it show is she did an ancestry dna test, I have already done one and had many 4th cousin results etc. We believe we share the same father but unfortunately he died ten years ago and it is apparently quite difficult to test half siblings.

  9. Hello,

    Long story but I have autosomal matches with distant cousins with a slightly different surname spelling. We descend from two brothers. Our MCRAs are from 18th century Hessen, Germany and church records state the brothers were both illegitimate and the same name is given for the father. I had always assumed yDNA tests would prove out the “brother theory”, however, they have not. So a working theory is that they were half brothers. Within the last week, Ancestry (Thru Lines) began to change not only my cousin results, but that of my father’s and those of distant cousins on my side of the family. Instead of showing cousin matches from the other brother under both MCRAs, they are removing the matches from the male MCRA and putting them under the female MCRA and calling them “Half” cousins. Wow! This is a shock, but it does go to the half brother theory. So is what I am seeing is Ancestry’s FIR predictions being able to confirm distant cousins (even some 5th cousins) as half cousins?

    Thanks!

    1. DNA can only distinguish full from half relationships if both individuals have tested. That is, we could use FIRs to tell if the German men were full vs half brothers if *they* had tested, but not when comparing their descendants.

  10. My dad& his “brother, Larry “, in their early 70s now.never a doubt they were full siblings& have a half sister(My grandma was widow w/ a baby, this half aunt i refer to, when she met my grandpa& later had my dad& my “uncle”) but my aunt, in her late 80s& Deathbed confession, says my grandpa( her step dad)raped her@ 16& she became pregnant with “Larry” & gave him to my grandma to raise alongside my dad, as hers& my grandpa’s child. My dad& “Larry”, BOTH did dna& show FULL siblings@ 2269 cM. But if these things are true, they’d be half siblings. Only sharing a father. However, since “Larry’s” mom, is my dad’s half sister(thru my grandma’s side) it SEEMS, my dad is “Larry’s ” half uncle AND his half brother, by sharing the same father…..Could being related 2 ways, add more dna or skew data?&predict full sibling relationship?& affect how “Larry” , shows up to match other family?

    1. Yes, being related two ways (paternal half brothers and maternal half uncle/nephew) would cause them to share more than typical half siblings and would also cause them to have FIRs, which might cause Ancestry to predict them as full brothers.

      Two things to check:
      1) In your father’s match list, click the little “i” in a grey circle beside the amount of DNA he shares with his brother. You’ll get a pop-up showing the possible relationships and how likely each one is. What does it say?
      2) Can you find matches in your uncle’s match list who would be related through your grandmother’s first husband? They could provide evidence that Larry was your half-aunt’s biological son.

  11. I can try. I’m no expert on Visual Phasing but I learned a little last year to help solve the problem I mentioned.

    1. Click on either the little “i” in a grey circle beside the amount of shared DNA (on a computer) or the amount of shared DNA itself (on a tablet/phone) and you’ll get a pop-up that tells you the percent chance of that relationship. What does it say?

    1. Perfect! You’re full siblings who just happen to be on the lower end of the scale. If you fought a lot as kids, you can blame your DNA. 🙂

  12. It says 100% sibling, nut it also says “Other possible relationships between people should also be considered,”

  13. Then Blaine Bettinger’s chart Is wrong? He has overlap built in. This was a comment by DNAdetectives between admin and another member about my match to my brother. But she isn’t 100% certain.
    “This appears to be a case of 3/4 siblings. Fathers being brothers. The mother is not related to them. They have FIR, which was the reason Ancestry labeled them as full siblings, causing the confusion. Half siblings do not have FIR. And the numbers were just above the average for 3/4 siblings. Their nearest paternal match was a second cousin. Both fell right in range and it was the expected relationship. I have not seen his whole…”
    I’d really like to get the full truth. Please offer guidance?

    1. We don’t have a whole lot of data on how Ancestry assigns percentages for these cases, but I know of one instance where the siblings were 3/4 and the percentages were 25% full and 75% half. That’s only one data point, though. To know more, I’d recommend tallying the amount of FIRs at a site that shows them (like 23andMe or GEDmatch).

    1. I am so confused. I have double endogamy. Mother is 100% Ashkenazi and my father is from a small town in Jalisco Mexico of only 300 people. Everyone in the town is related and they somehow ended up with some ancient Jewish DNA. Is it normal to only match my sibling at 2251cm?

      1. Because your parents were from different endogamous populations, the endogamy won’t affect how much you share with your sibling. A DNA match of 2251 cM is just about equally likely to be full sibling or half sibling. The key in telling them apart is whether you share FIRs. Where did you test?

  14. So I manage a bunch of my family’s kits on Ancestry including both my parents, most of my Dad’s siblings, and most of my full and half siblings. My Dad and his full brother both married the same woman and each had children with her. I manage the kit of one of those children, my half sister T. Recently, a child from my uncle and this woman, I’ll call him B, tested on Ancestry. B and I are full regular 1st cousins and share 933 CM. My Dad, B’s uncle and T’s father, share 1764 CM. T and her father share 3458 CM. So nothing unusual going on and no endogamy in the family. B and T are 3/4 siblings. On Ancestry, B and T share 2193 CM. Ancestry lists them as “Full Siblings”. When you click on the gray “i” circle, it says they are 100% siblings. Perhaps B and T share more FIR segments than typical 3/4 siblings? Maybe my Dad and my uncle are really close DNA full siblings? Unfortunately, I don’t think I can get B to put his kit out on Gedmatch to investigate it any further. I will try, but I think B just did the kit to find out ethnicity and isn’t all that interested in genetics.

    1. I’ve only seen one other case of confirmed 3/4 sibs at Ancestry, so I don’t have a good comparison sample to refer you to. They were pegged at 25% full/75% half.

    2. Sarah story is similar to mine as well but we have endogamy and only 2251cm. We have had a lot of people tested and only the one I Share with my brother seems off.

  15. My brother and I both used 23andMe. The results are confusing. It states that we are half siblings. We share 28.1% dna and 2090cM. We also share 58 segments of half identical regions of dna and 10 segments of fully identical regions of dna. I did an actual paternity test between my brother’s dad and me and he is not my father. So why do my brother and I share fully identical regions of dna if we are half siblings?

    1. Are the fully identical regions small? That is, just a few centimorgans each? Are you and your brother from an endogamous population?

  16. The fully identical regions are 20.63, 18.28, 16.71, 15.3, 12.79, 10.37, 7.78, 7.14, 6.56. I don’t think we are from an endogamous population. My brother and I have a half sister, she shares no fully identical regions with either of us. Another thing I find interesting is that our 23andme report shows that my brother and I have lots of relatives in common that are not related to our sister. Thanks for your help. I’m searching for my birth father and I am completely overwhelmed by all the information.

  17. My sister and I used AncestryDNA it says we match at 2294cm across 63 segments and when we click it, it says 100% sibling. Thing is our father said he was not my father, that my uncle was my father (he is now deceased) and testing other family is not an option for us. So we uploaded to GEDmatch and looked at the FIRs and I am so confused bc there is no data to help with 3/4 siblings and inherited by descent FIRs. So we have some matching FIRs, only 23.7% but many are small. I will list them below:

    29.8, 20.4, 99.0, 40.6, 27.0, 25.7
    none
    38.9, 11.3
    12.6, 13.3, 27.3
    58.9, 12.1, 34.1
    9.7, 13.1
    40.9, 9.1, 45.2
    none
    27.4
    14.0
    12.6
    none
    14.0
    none
    42.4, 16.5, 20.1
    24.1, 20.0, 15.8
    13.6
    none
    17.8
    22.0
    none

    Our HIRs are 67.074%

    Your opinion on if we are full or 3/4 siblings and for inherited by descent FIRs how small of a segment should you not consider when running a FIR comparison >7cm, >10, >20, etc…?

    Thank you for helping us clarify things.

  18. Can two brothers share enough common DNA to make a 1st cousin appear as a half sibling? My father’s name was supposed to be C***** and the children of his brother are appearing as half siblings. The cMs on one is 1911 and the others are 1400 -1530. Along that whole family line the cM are elevating relationships, second cousin are appearing as first cousins. I am confused and unsure what to believe.

  19. I took an Ancestry DNA test and it matched me with a “close relative”. We share 2,082 cM across 60 segments. What are the chances she is my half-sibling?

    1. Very high. The only other options are full aunt/niece or grandmother/granddaughter. With that amount of shared DNA, there’s even a slight chance she’s a full sibling. If that were the case, though, Ancestry would put her in the Full Sibling category.

  20. Thank you so much. We ordered sibling dna tests and are gonna have it tested. I will let you know the results! Thank you for your help!

  21. I did an ancestry test a while ago and then found out about 3 months ago that my nephew did an ancestry test that came up with 1780 cms and 83 segments shared with me but the result said he was my half nephew and my brother was a half sibling fathered by my fathers sibling. Quite upsetting to find this out. The sibling issue is confusing also because I don’t know which one of us is to which brother. So the father I grew up with I’m estranged from and his brother died a couple of years ago, and I’m estranged from my brother also.(not a happy family) How do I find out who is my father? If you are able to give me some guidance, I would appreciate it very much

  22. The ancestry results said my brother was my half brother through my mother and my brother was my cousin through his father. It inferred that my mother had 1 child each from the 2 brothers. It describes my nephew as close relative-cousin

    1. Ancestry matching doesn’t report relationships that way. Are you seeing that in ThruLines? If so, it may be because of an issue in your trees.

  23. Not all of us are biologists and leave the experience with those more qualified, hence paying to have DNA tests to verify relationships and/or health. Can’t help notice that it’s not all adopted kids discovering half-siblings but even those within the same household are discovering such now.
    I am fascinated with all of this and confused by it also. I had a DNA test done and was told I had a cousin and great nephew but my issue is trying to determine it I am linked to Tom junior or senior. Party found who I’m linked to through his DNA, am uncertain if he’s my great nephew or his son. They are not too forthcoming discovering that their line produced an illegitimate child or two.

  24. No kidding. Can be very discouraging but still intend to plod on as somewhere within the research an answer will emerge. Can’t help wonder why they’d do a DNA test knowing that there may be half-siblings or something inconsistent within their heritage, especially since medical/health issues and records have further advanced over the years and we’ve been given the opportunity to determine medical history and our forefathers lives through the years.

  25. I have a friend who is adopted. Through her cousin matches and research, we think we have identified her birth father. She contacted the man’s daughters and one agreed to submit their DNA to Ancestry. They shared 1286 cm. Ancestry listed them as first cousins, but when you look into it further, they say there is a 51% chance they are half siblings. What are your thoughts?

    1. I’d want to compare their X chromosomes. If they match all the way across, that lends more weight to the idea that they share a father. You can compare the X at 23andMe, FamilyTreeDNA, and/or GEDmatch.

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