We can now easily differentiate between 3/4 siblings and full-siblings over 94% of the time
Update 27 Apr. 2023: Please note that there are no restrictions on sharing screenshots of any articles from this site, including this one. The only restriction applies to submitting data from relationship predictors to surveys that compare group probabilities of one tool to individual probabilities of another tool. The reason is that group probabilities will always be higher than the individual relationships that make up the groups, as explained in this article and as apparent when using this tool.
Three-quarter siblings have been confounding genetic genealogists for years. The problem is that their shared DNA overlaps significantly with other sibling types. And, while you can usually tell the difference between 3/4 siblings and half-siblings pretty easily, it was thought to be very difficult for full-siblings. Until now.
Click here for relationship predictions that include 3/4 siblings and other double cousin relationships, or try the predictor that uses the number of segments along with total cMs.
But first it’s important to understand exactly what a 3/4 sibling is and what types there are.
Here’s a common scenario that has resulted in 3/4 siblings: After a man’s wife passes away, he marries her sister. If both women had a child with that man, those children are 3/4 siblings to each other. It can also happen where 3/4 siblings share a mother and the fathers are full-siblings. Here are the ways in which you could end up with a 3/4 sibling:
- Your father had a child with your maternal aunt/niece
- Your mother had a child with your paternal uncle/nephew
- Your father had a child with your maternal half-sister
- Your father had a child with your maternal grandmother (opposite perspective of the immediately above type)
- Your mother had a child with your paternal half-brother
- Your mother had a child with your paternal grandfather (opposite perspective of the immediately above type)
In none of these scenarios are the parents related to each other. This is a type of double relationship without pedigree collapse. I’ve already listed averages and ranges for 3/4 siblings here.
A common refrain among genetic genealogists is that 3/4 siblings can share DNA anywhere between half-siblings and full-siblings. That is far from true. 3/4 siblings have their own range, and while it is between the other two, a 3/4 sibling won’t have shared DNA as low as the minimum for half-siblings or the maximum for full-siblings.
When discussing 3/4 siblings, I usually mention that half-identical regions (HIR) are the worst way to differentiate between siblings types. Unfortunately, this is the default metric reported at every company except 23andMe, and it’s the only metric available in centiMorgans (cM) reported at AncestryDNA and MyHeritage. Fully-identical regions (FIR) are a much better tool for this task. While half-siblings share zero FIR, 3/4 siblings share 12.5%, on average, and full-siblings share 25%, on average. This is what’s normally reported in scientific journals. It consists of both the HIR and FIR segments. Since the usual solution to see FIR is to upload to GEDmatch and since cM are different at each site, GEDmatch numbers will be used in this article unless otherwise noted. A third and final metric that will be discussed here is identical by descent (IBD) sharing.
Above I claimed that I’ve found a way to tell the difference between 3/4 siblings and full-siblings over 94% of the time. Let’s see how well these different metrics do.
I’m going to start by finding how well the HIR metric predicts sibling types. I used a dataset of 500,000 full-siblings and 500,000 3/4 siblings. Each were labeled with the true relationship. Then I guessed at a value of HIR cM that would split them as equally as possible. I thought that it would be between 32.8% and 36%, since that’s where the lower limit of the 95% confidence interval for full-siblings is near the upper limit for the 95% confidence interval for 3/4 siblings. Anything over the limit would get a predicted label of “full-sibling” and anything under the limit would get a predicted label of “3/4 sibling.” I would then count how many of the predicted labels matched the true labels and divide by one million data points. After guessing at a value for the limit, I would try other values and see if the fraction of correct predictions improved or got worse, continuing in the direction of improvement until finding the best value. The best HIR predictions came from a cutoff of 2,476.0 cM. Using this value, one will predict the right sibling relationship 89.74% of the time, which is surprisingly good. This value roughly splits the zone of overlap shown in Figure 1.
Figure 1. HIR cM vs. FIR cM for full-siblings and 3/4 siblings. The red line shows the best HIR cutoff value to use in order to differentiate between full-siblings and 3/4 siblings.
Next up is the FIR metric. You can see FIR at 23andMe, or at GEDmatch by clicking the “Show only Full-Match (FIR) segments” checkbox in the One-to-one comparison tool. The methodology was the same as for HIR, only I anticipated a value between 16.4% and 20% FIR. The best prediction was made at 645.1 cM, which appears to split the overlap zone horizontally in Figure 2. Guessing that 645.1 cM or greater is a full-sibling match and that a lower value is a 3/4 sibling match will be correct a whopping 94.39% of the time.
Figure 2. HIR cM vs. FIR cM for full-siblings and 3/4 siblings. The red line shows the best FIR cutoff value to use in order to differentiate between full-siblings and 3/4 siblings.
It’s hard to imagine an improvement on the last method. But, then, Figures 1 & 2 show that a diagonal line would probably be a better predictor than a horizontal line, which could be obtained by using a total IBD cutoff rather than HIR or FIR. It turns out that IBD sharing does provide us with more accurate results. A cutoff value of 3,126 cM will predict the right relationship 94.49% of the time. That IBD amount is shown by a diagonal line in Figure 3.
Figure 3. HIR cM vs. FIR cM for full-siblings and 3/4 siblings. The red line shows the best IBD cutoff value to use in order to differentiate between full-siblings and 3/4 siblings.
It seems as though a curved line—maybe one with a backwards “s” shape—would make an even better prediction. But I’m going to save that for another day. There is one more method I’d like to explore, though. K-means is a powerful machine learning technique used for clustering. I ran a k-means program on all one million data points. It turns out that it does only slightly better. The K-means algorithm predicts the correct sibling type 94.56% of the time. This is a slight improvement over using an IBD cutoff, but it probably isn’t worth it for a person to do a k-means cluster on one million data points and then check to see which cluster a match falls into.
Figure 4. HIR cM vs. FIR cM for full-siblings and 3/4 siblings. Data points have been clustered by the k-means algorithm. Since the labels are predicted, not all labels are correct.
In case anyone finds this helpful, these are the coordinates of the centroids (cluster centers) in Figure 4:
- Full-Sibling: (HIR, FIR) = (37.72%, 25.56%) = (2,706.0 cM, 917.0 cM)
- 3/4 Sibling: (HIR, FIR) = (31.4%, 12.7%) = (2,251.9 cM, 453.8 cM)
Another method that has been used to differentiate between the two relationships is a log likelihood ratio. That and the k-means clustering method might not be accessible to most genetic genealogists. However, the cutoff values and the tools used here are very accessible.
Table 1 below summarizes the best cutoff value to use along with the accuracy achieved for each method.
Table 1. Comparison of all three metrics discussed above along with the accuracy for each method. †Please remove any X-DNA from 23andMe data before doing the analysis described here as X-DNA only confounds relationship prediction. *FIR percentages are reported as a proportion of one copy of the genome, unlike HIR percentages. To convert from FIR percentage to cM at GEDmatch, for example, one would have to multiply 17.985% by 7,174 cM, divide by 100%, and also divide by 2.
Getting predictions with over 94% accuracy is great, but if a value is very close to the cutoff then there’s a lot less certainty. For this reason, I always recommend plugging these values into a double cousin relationship predictor to see exactly how likely the options are. The cutoff values shown above are good for a quick check—maybe some people will even memorize that 2,399 cM is the cutoff for AncestryDNA and will get an idea of what to expect before using the relationship predictor. And hopefully you have an IBD value close to one of the means, as shown in Figure 5. (These screenshots were taken from an older version of the double cousin predictor.)
Figure 5. Results displayed in the multiple cousin relationship predictor when the IBD value equals the average for the most likely relationship: full-sibling on the left and 3/4 sibling on the right.
If you share anything near the average with your 3/4 or full sibling, you’re going to get a very strong prediction for the correct relationship. That’s relieving. We see that relationship predictors really can tell the difference. And we’ve learned that choosing the most likely option each time will result in over 94% accuracy in differentiating between full-siblings and 3/4 siblings!
DNA-Sci — advancing the science of relationship predictions. Feel free to ask a question or leave a comment. And make sure to check out these ranges of shared X-DNA, shared atDNA percentages, and shared atDNA centiMorgans. Would you like help visualizing how much DNA full-sibling share? Or, try a tool that lets you find the amount of an ancestor’s DNA you cover when combining multiple kits. I also have some older articles that are only on Medium.
What is the probability. A full sibling in the sample of 500,000 is truly a full sibling vs probability that a 3/4 sibling is correct?
Might some of the full sibling sample be undisclosed 3/4 siblings.
Would expect more “secrets” to remain in the full sibling sample.
Ps bravo for normalizing the power of the samples by picking 500,000 each.
Thank you for your question. The probability that a full-sibling or 3/4 sibling pair is mislabeled in this case is zero. That’s because the data came from simulated pedigrees. When testing the methods, sometimes the data will be mislabeled, but then can be checked against the known relationship type.
After trying to read all you have put out for us to read and understand I still am clueless on understanding. I am the daughter of first cousins and my fathers mother and my mothers father are siblings . I had my half brother do his DNA and he came is at 1870 cM’s . Are we 3/4 siblings or not?
Thank you for any help
3/4 siblings are half-siblings plus 1st cousins. Your half-brother and you are instead half-siblings plus 2nd cousins. If you were half-siblings plus your other parents were full-siblings, then you’d be 3/4 siblings. Instead, your other parents parents are full-siblings. Your cMs suggest that you’re 3/4 siblings: https://dna-sci.com/tools/orogen-mult-unw/
But I suspect that you share extra DNA with your half-brother (plus 2nd cousin) because your parents are 1st cousins to each other.
I think the following scenarios would result in cousins who are as related genetically as 3/4 siblings: two full non-identical brothers each have children with identical twin sisters, or 2 full non-identical sisters each have children with identical twin brothers. Did I get the “genetic math” right?
Yes, you’re correct. Some of these situations can be difficult to figure out. A good trick to make it easier–maybe this is what you did–is to assume that the identical twins are the same person. I can usually picture a scenario pretty clearly after doing that.
The ranges of shared DNA in this case should be the same as for the “horizontal” types of 3/4 siblings (first two listed) found here: https://dna-sci.com/2020/05/15/how-much-of-an-ancestors-or-relatives-dna-do-you-have/
Extending this concept a bit further, in a scenario where sibs share say a common father but the mothers are half-sisters, wouldn’t we expect the HIR and FIR amounts to lie between the values for half sibs and 3/4 sibs? I like to think of this scenario as 5/8 sibs.
Sorry I just can’t follow. Can you please tell me the likelihood of my mom and her brother being 3/4 siblings instead of full siblings. the rumor is their mother had relations by two brothers but the DNA on ancestry says they are full siblings. here is their info on Ancestry 36% – 42% shared DNA 2051 cm across 51 segments. thank you so much
Ancestry always labels both relationships as “sibling.”
The only thing we can go by at this point is the total half identical region (HIR) cMs: https://dna-sci.com/tools/orogen-mult-unw/
It tells you that there’s an 86.6% chance of being 3/4 siblings and a 1.7% chance of being full-siblings.
But if you can get the kits to GEDmatch you’ll get a better prediction. You can run the one to one comparison, write down the amount of HIR cMs, go back one step and check the box to show full-identical regions (FIRs) only, and then add that to the HIR amount to get total IBD.
You can then enter that value into the 23andMe box in the tool I linked to above. 23andMe is the only company that uses total IBD by default.
If it still shows a very high chance of 3/4 siblings, then that’s even more definitive than what we have now.
I am uncertain as to whether I am entering my data correctly in your tools for predicting relationships. My brother and I are trying to figure out who our biological parents are. We have the same mother, but are confused as to whether we have the same father or if our fathers are brothers. We have submitted our DNA to both Ancestry and 23andme. On Ancestry, we share 2374 cm across 46 segments, and it states that we have 49% shared DNA. On 23andme, we share 3121 cm across 45 segments and it states that we have 41.95 shared DNA. Any help in deciphering this information would be greatly appreciated. Thank you.
The best prediction at this time will come from entering 3,121 cMs into the 23andMe input box for the double cousin predictor: https://dna-sci.com/tools/orogen-mult-unw/
It says there’s an 80% chance of you being full-siblings. Unfortunately, that isn’t definitive. Something else you can do is check the X Chromosome at 23andMe in the Advanced DNA Comparison tool. If you’re full-siblings, you’ll likely share 181.84 cMs of X-DNA that’s labeled “Half.” If the amount of Half identical X-DNA is much less than that, you can’t share the same father. If you share 181.84 cMs of X-DNA, you likely share a father. I’m interested to hear what you find.
Thank you for suggesting the Advanced DNA Comparison tool. My brother and I share 168.63 cMs of X-DNA. I am thinking at this point that we may be 3/4 siblings. I have another puzzle to solve, which also leads me to believe that our fathers may be brothers, rather than us having the same father. On Ancestry DNA, I recently discovered that I share 1863 cMs across 35 segments, 27 percent shared DNA with a paternal relative and the relationship suggests that we are half siblings, but my brother only shares 939 cMs across 29 segments, 13 percent shared DNA with him. I believe my brother said that his relationship showed them as being cousins. That is not the end of the puzzle solving. We have a sibling that has passed, so we do not have his DNA to compare to, but his daughter, on Ancestry, shares DNA on both sides of our family, so we are assuming that one of our possible fathers is her father’s biological father as well. She shares 964 cMs with the paternal relative mentioned above. My brother shares 1525 cMs, 45 segments, 22% with our niece, I share 1512, 44 segments, 22%. She is interested in finding out who her grandfather is, as well as we are. Of our two possible fathers, they have only one child who has submitted their DNA on Ancestry. No other children have submitted DNA and shared that information on Ancestry or 23andme. Our parents, aunts and uncles have all passed, so we are limited to what we have on the DNA sites. If I haven’t confused you too much, would you have any suggestions on how we can solve these mysteries? It would be greatly appreciated.