Philip Gammon, our statistician friend has been working with crossover simulations again in order to tell us what we might expect relative to how much DNA we actually inherit from grandparents and great-grandparents.
We know that on average, we’re going to inherit 25% of our DNA from each grandparent – but we also know in reality that’s not what happens. We get more or less than exactly 25% from each person in a grandparent pair. It’s the total of the DNA of both grandparents that adds up to 50% for the couple.
How does this work, and does it make a difference whether we inherit our grandparent’s DNA through males or females?
Philip has answers for us as a result of his simulations.
DNA Inheritance from Grandparents
Philip Gammon:
When we consider the DNA that we inherit from our ancestors the only quantity that we can be certain of is that we receive half of our autosomal DNA from each parent. This is delivered to us in the form of the 22 segments (i.e. chromosomes) provided by our mothers in the ova and the 22 segments/chromosomes provided by our fathers in the sperm cell. Beyond parent-child relationships we tend to talk about averages. For instance, we receive an average of one quarter of our DNA from each of our four grandparents and an average of one-eighth of our DNA from each of our eight great-grandparents etc.
These figures vary because our parents didn’t necessarily pass on to us equal portions of the DNA that they received from their parents. The level of variation is driven by the number (and location) of crossover events that occur when the ova and the sperm cells are created.
The statistics relevant to the recombination process were discussed in detail in a previous article (Crossovers: Frequency and Inheritance Statistics – Male Versus Female Matters). With the availability these days of abundant real data from direct-to-consumer genetic testing companies (such as the 23andMe data utilised by Campbell et. al. in their paper titled “Escape from crossover interference increases with maternal age”) we can use this information as a basis for simulations that accurately mimic the crossover process. From these simulations we can measure the amount of variation that is expected to be observed in the proportions of DNA inherited from our ancestors. This is precisely what I have done in simulations run on my GAT-C model.
Before looking at the simulation results let’s anticipate what we expect to see. The previous article on crossover statistics revealed that there are an average of about 42 crossovers in female meiosis and about 27 in male meiosis. So, on the set of 22 chromosomes received from our mothers there will have been an average of 42 crossover locations where there was a switch between DNA she inherited from one parent to the other. That means that the DNA we inherit from our maternal grandparents typically comes in about 64 segments, but it won’t necessarily be 32 segments from each maternal grandparent. Chromosomes that experienced an odd number of crossovers contain an even number of segments (half originating from the grandmother, the other half from the grandfather) but chromosomes with an even number of crossovers (or zero!) have an odd number of segments so on these chromosomes you must receive one more segment from one grandparent than the other. And of course not all segments are the same size either. A single crossover occurring close to one end of the chromosome results in a small segment from one grandparent and a large segment from the other. All up there are quite a few sources of variation that can affect the amount of DNA inherited from grandparents. The only certainty here is that the amount inherited from the two maternal grandparents must add to 50%. If you inherit more than the average of 25% from one maternal grandparent that must be offset by inheriting less than 25% from the other maternal grandparent.
The above chart shows the results of 100,000 simulation runs. Excluding the bottom and top 1% of results, 98% of people will receive between 18.7% and 31.3% of their DNA from a maternal grandparent. The more darkly shaded region in the centre shows the people who receive a fairly even split of between 24% and 26% from the maternal grandparents. Only 28.8% of people are in this region and the remainder receive a less even contribution.
On the set of 22 chromosomes received from fathers there will have been an average of around 27 crossovers so the DNA received from the paternal grandparents has only been split into around 49 segments. It’s the same amount of DNA as received from mothers but just in larger chunks of the grandparent’s DNA. This creates greater opportunity for the father to pass on unequal amounts of DNA from the two grandparents so it would be expected that results from paternal inheritance will show more variation than from maternal inheritance.
The above chart shows the results of 100,000 simulated paternal inheritance events. They are more spread out than the maternal events with the middle 98% of people receiving between 16.7% and 33.3% of their DNA from a paternal grandparent. Only 21.9% of people receive a fairly even split of between 24% and 26% from each paternal grandparent as shown by the more darkly shaded region in the centre.
To help with the comparison between maternal and paternal inheritance from grandparents the two distributions have been overlayed on the same scale in the chart above. And what are the chances of receiving a fairly even split of grandparents DNA from both your mother and your father? Only 6.3% of people can be expected to inherit an amount of between 24% and 26% of their DNA from all four grandparents.
Now I’ll extend the simulations out to the next generation and examine the variation in proportions of DNA inherited from the eight great-grandparents. There are effectively four groups of great-grandparents:
- Mother’s maternal grandparents
- Mother’s paternal grandparents
- Father’s maternal grandparents
- Father’s paternal grandparents
The DNA from group 1 has passed to you via two maternal recombination events, from your mother’s mother to your mother, then from your mother to you. On average there would have been 42 crossovers in each of these recombination events. Group 4 comprised two paternal recombination events averaging only 27 crossovers in each. The average amount of DNA received along each path is the same but along the group 1 path it would comprise of more numerous smaller segments than the group 4 path. Groups 2 and 3 would be somewhere between, both consisting of one maternal and one paternal recombination event.
The above chart shows the variation in the amount of DNA received from members of the four groups of great-grandparents. 25,000 simulations were performed. The average amount from any great-grandparent is 12.5% but there can be considerably more variation in the amount received from the father’s paternal grandparents than from the mother’s maternal grandparents. Groups 2 and 3 are between these two extremes and are equivalent. It doesn’t matter whether a paternal recombination follows a maternal one or vice versa – the end result is that both paths consist of the same average number of crossovers.
The table below shows the range in the amount of DNA that people receive from their great-grandparents. The bottom and top 1% of outcomes have been excluded. Note that these are based on a total of 3,418 cM for the 22 autosomes which is the length observed in the Campbell et. al. study. The average of 12.5% of total DNA is 854.5 cM:
| Group | 1st percentile | 99th percentile |
| Mother’s maternal grandparents | 522 cM | 1219 cM |
| Mother’s paternal grandparents | 475 cM | 1282 cM |
| Father’s maternal grandparents | 475 cM | 1281 cM |
| Father’s paternal grandparents | 426 cM | 1349 cM |
As a matter of interest, in each of the 25,000 simulations the amount of DNA received from the eight great-grandparents were sorted into order from the highest cM to the lowest cM. The averages of each of these eight amounts were then calculated and the results are below:
On average, a person receives 1,129 cM from the great-grandparent that they inherited the most of their DNA from and only 600 cM from the great-grandparent that they received the least of their DNA from. But none of us are the result of 25,000 trials – we are each the product of recombination events that occurred once only. The above chart shows the average or typical variation in the amount of DNA received from the eight great-grandparents. Half of people will have experienced more variation than shown above and half of people will have experienced less variation.
Could you have received the same amount of DNA from all eight grandparents? Of course, it is possible, but it turns out that it is extremely unlikely. The average is 12.5% (854.5 cM) so anything between 12% (820.4 cM) and 13% (888.7 cM) could be considered as being close to this figure. The results reveal that this did not occur in any of the 25,000 simulations. Not one person received amounts between 12% and 13% from all eight great-grandparents.
Widening the criteria, I observe that there were 13 instances in the 25,000 simulations where people received between 11.5% and 13.5% of their DNA from all eight great-grandparents. That is still an extremely rare occurrence. Expanding the range further to between 11% and 14% saw a total of 126 instances, but this still only represents about half a percent of all observations. I think that we just have to face the fact that unless we are an extremely rare individual then we will not have inherited close to equal amounts of DNA from our eight great-grandparents.
Now, back to Roberta.
Thanks Philip.
Now we see why we might not inherit the same amount of DNA from our grandparents and great-grandparents.
We Don’t Have Equal Numbers of Matches on Tree Branches
This also might explain, at least in part, why people don’t have the same number of DNA matches on each branch of their tree.
Of course, other reasons include:
- Uneven family sizes
- Fewer or more cousins testing on different branches
- Recent immigration meaning there are few people available to test
- Family from a region where DNA testing and/or genealogy is not popular
- Endogamy which dramatically increases the number of people you will match
Real Life Example
In our real-life example, two grandchildren are fortunate to have three grandparents and one great-grandparent available for matching.
For comparison purposes, let’s take a look at how many matches each grandchild has in common with their grandparents and great-grandparent.
The line of descent is as follows:
Both end of line testers are female children.
The transmission path from their great-grandmother is:
- Female to their paternal grandmother
- Female to their father
- Male to female tester
The transmission path from their maternal grandfather is:
- Male to their mother
- Female to female tester
The transmission path from their maternal grandmother is:
- Female to their mother
- Female to female tester
This first chart shows the number of common matches.
| Matches | Grand 1 | Grand 2 | GGF | GGM | Grand 3 | Grand 4 |
| Female 1 | absent | 1061 | absent | 238 | 529 | 1306 |
| Female 2 | absent | 1225 | absent | 431 | 700 | 1064 |
It’s interesting that the matches in just 3 generations to the great-grandmother vary by 55%. The second tester has almost twice as many matches in common with her great-great-grandmother as she does the first tester. There a difference in the earlier generation, meaning matches to Grand 2, but only about 23%. That difference increased significantly in one generation.
The second chart shows the total number of matching cM with the matching family member.
| Total cM | Grand 1 | Grand 2 | GGF | GGM | Grand 3 | Grand 4 |
| Female 1 | absent | 1688 | absent | 713 | 1601 | 1818 |
| Female 2 | absent | 1750 | absent | 852 | 1901 | 1511 |
We can see that the amount of DNA inherited from a grandparent does correlate with the number of matches to that grandparents. The more DNA shared, of course the better the chances of sharing that DNA with another person. However, multiple factors may be involved with why some people have more or fewer matches.
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Thanks Roberta and Philip once again for a helpful article.
Thank you for posting this. Without following every step in the calculations, I appreciate the reasons for the uneven spread in my DNA matches in different lines.
Great article. This helps explain why I have some 5-7th maternal cousins matches on FTDNA FamilyFinder (FF), but don’t match a known 4th cousin.
I enjoy from ALL your posts, and learn from all of them. I am an only child, and my parents and grandparents are long dead. I am working to trangulate my 3rd, 4th, and even 5th cousins. The Shared cM chart helps, but as you mentioned, the dearth of records (in the south), the endogomy that the isolation of the westward movement forced, as well as my lack of living relatives makes the job difficult!
Again, thanks!
I was also wondering about this. I have apparently inherited almost 25% of my Dad’s paternal grandma’s DNA. I actually share the same amount of her DNA with a couple of my Dad’s first cousins that are closer in age with me. They were also surprised by it. The rest of my DNA tests (I tested with several companies) support this also. My brothers haven’t tested yet.
With endogamy, I guess it would be possible for the amount of shared DNA from either the two paternal grandparents or the two maternal grandparents to add up to more than 50% if a set of grandparents came from the same community?
Fascinating read…thanks Roberta!
Thank you for this information. I understood we all do not receive equal portions from each grandparent, or great grandparent, but this puts it into statistical form what to expect or what is likely. I have been disappointed that my predicted ethnicity included so little German, but my strong German ancestry is on my father’s paternal grandparents!
That may just be a function of the ethnicity predictions and not what you inherited.
I also expected a lot more German than I got (I have German on both sides), but some it is apparently in the Iberian percentage – with movement of our ancestors.
Thank You for this most interesting topic. Clears up a lot of misinderstandings 💞
Of course everything is different when the parents are first cousins, or there are other links between family lines.
Like a family lineage full of Indogomy!
The total cMs in each generation never changes – it always has to add up to 100%. But the number of segments is skewed for female vs male transmissions. There is a maximum disparity from our parents, but with each succeeding generation going back, the differences tend to even out. We’ll usually see the differences in our grandparent segments, and often in great grandparent segments, but I don’t think we’ll see much difference in the 6C to 8C levels, where we have most of our Matches in a genealogical timeframe. Again, the total cMs in any generation will be the same, while the number of segments increases (by about 69 additional segments in each generation). At some point the segments are so small that we won’t see shared DNA segments with Matches, and cannot identify those segments easily.
Absolutely fascinating! This could partly explain why I have so few Smith matches, though I have a Smith grandfather.
Turns out, this concept is more than fascinating – it’s helping greatly in my research. I did more digging after reading this post the first time. Though I knew what meiosis was, I watched a couple of videos that helped me better get my hands around it.
I’ve been looking more closely at matches I have added to DNA Painter, and have been able to identify crossovers in mine and my mom’s DNA. This has helped me better qualify which common ancestor a given segment likely comes from, and allows me to better focus research on those lines.
Thanks so much for sharing this!
Hello Roberta,I have quite a high DNA match with 2 different people one proved to be a half second cousin with whom I share a great grandfather,the other with slightly higher DNA has caused me a problem because although there is one common surname,that is found in only one greatgreatgreat grandmother and there is no proof that proves a relationship because the name is common in the area.Nothing else no common surnames back to at least 6 generation s back.This is very frustrating for both of us.
Assuming our tree information is correct and there is no good reason to doubt this,what reason could you suggest for this mystery?
Thanks
Peter Radford
United Kingdom
Hello. Thank you for this. I only have one question – no need to know percentages. My grandfather was German(long deceased). His mother was German(long deceased) Do i have German DNA and if I do …is that from my grandfather’s side because his mom was German? I find this confusing. Many thanks,
Yes, if I understand the question correctly. From your description, your grandfather was half German. And if his other parent was from a border country, it may be impossible to tell the difference. Country boundaries are political, not genetic.
I like your answer, which straightens a ton of misgivings many have, re: nationality, which DNA doesn’t recognize. I’m looking for a different answer to a related (pun intended) scenario: I’d like to know how many cMs I share with 4th great grandparents? Thanks
Guy Baker
Very useful post. When I reconstructed what i received from each grandparent using myself and my 2 sisters DNA and using matching data from relatively close relatives I ended up with about 41 crossovers on my mothers side and only 19 or 20 on my fathers side, still working out a few kinks where don’t have much matching data. but its pretty accurate. So on my mothers side because of a lot of crossovers received close to 25% from each of the grandparents through her, I didn’t work it out more exactly as what more or less expected. On my fathers side it was 17.5% and 32.5%, which I was reading your post to see whether this was even reasonable. One grandparent on that side accounts for, at least, 1134 cM, which I can verify, for the others I don’t have enough matching data to know, although one falls in the middle and of the other two, at least one of them must be fairly low. Anyway your post does show that these results do fall within reasonable statistical probability.
Hello,Thank you so much for this topic.
I’ve been trying to understand where the substantial amount of Iberian ethnicity (upper 20%) is coming from in my aDNA test(s). Could the Iberian come from my paternal Grandparents. They were 2nd cousins through the d’aragon line.
My paternal Grandfather’s Grandmother was Marie d’aragon. My paternal Grandmother’s Grandfather was Guillaume d’aragon. Marie and Guillaume were brother and sister. Would this show a high % amount of Iberian in my ethnicity?
My d’aragon line comes from my 7th Great Grandfather François d’aragon dit Lafrance. I do have a 8th Great Grandfather Pedro (Pierre) da Silva dit le Portugais from São Julião, Lisbon, Portugal. Pedro comes from my maternal lines.
Thank you….
I share 39% DNA with my maternal grandmother. My brother shares 50% with our maternal grandmother. We were both shocked at the amount of DNA shared with her. Is it possible based on what we’ve learned above?
That’s extremely high. Is there endogamy or are your grandparents related?
Thank you for this article. My question is about the amount of DNA that I inherited from my paternal Grandparents. They were 2nd cousins through the d’Aragon dit Lafrance line.
My paternal Great-Grandmothers madian name was d’Aragon. My average Spanish ethnicity is about 28% in 4 of my 5 DNA test. How can I get an accurate DNA % knowing my situation.
Thank you…..
Jeffrey Mitchell (de Saint Michel)
That’s a really tough question. Start with twice the amount in the range of second cousins. But then some of the DNA probably was contributed by both separate lines, so that DNA really doesn’t count twice. What you really need to know is the half identical regions and then the fully identical regions. But that’s probably more than you need for genealogy. So take the mid-range for second cousin and double it. That should be about right.
Thank you is much for your explanation.
Enjoy your site,hope you can make sense of this connection.
My mother a Talton -her grandfather CullenTalton his father Machlus Talton
My dad grandmother Matilda Talton HER father William Henry Talton
William Henry Talton and Cullen are bothers , Father is Machlus Talton to brothers.. So my parents are related..
my brother took a DNA test which shows German ancestry “Western part of Germany / Poland Iberian P, Spain and 1 % Jewish ” .\
MY %
Parent 1 I think my mother
England//Europe27 %
Sweden/Denmark12%
Wales 11 %
Parent 2 I think my dad
Eng./Europe 12%
S / Denmark 10%
Scotland 16
Ireland 9%
3 % Germanic E.
Our families lived in same area of NC “3 counties ” since 1700’s It seems this is because my parents are related / and lived in same area for so many years I am 39 % England / Europe 22% Sweden and Denmark.. THEN 16 % Scotland ” just from my dad”. seems a large % SURPRISE ONLY 3 % Germanic . My mom Wales large 11 % too because only 3 main areas I have inherited from my mom .. ALSO families lived in Va. ” same area”. 1600’s and wagon trains / groups moved to NC when land was given for free to settlers. SO they could be related more lines , is this why % so high same 2 areas.. I don’t know what to make of 16 % Scottish in my dad’s line seems a large %, for Germanic line 3%..SURPRISE to me..
Thank you for the article. It still all baffles me a bit and hope you can help me unravel a mystery.
I don’t appear as a DNA match to the my half 1st cousin 1-removed (from my maternal grandfather’s second marriage).
Neither do I share a DNA match with my 1st cousin 1x-removed (my mum’s father’s brother’s daughter).
I do, however, share a match with a 3rd cousin (mum’s father’s mother’s brother’s great grand-daughter).
It’s been suggested recently that my grandfather is not my biological grandfather; would these DNA result confirm despite a match through his mother’s sibling? Or is it still possible his was my mother’s biological father?
Many thanks
There is still a possibility. I would suggest clustering, like with Genetic Affairs, then identify the common ancestor of the clusters.
Hi, I have a 5th cousin on my maternal side, we share 5x Great grandparents and yet on my Ancestry DNA he is said to have 12cM and less than 1% DNA. Could we still be related or is one 5x Great grand parent not biological?
Yes, you could certainly share that much DNA due to a common ancestor.
Hi if 1% dna shows up in a person test what does that mean ? I have read that it could mean noise.If it is noise why would it be labeled as African, Native American of Portuguese ect.And can you inherit dna from a 6th Great Grandparent.
It could be noise, but it could also be quite valid. 1% is about 70 cMs. Also, yes, you can inherit DNA from a 6th great-grandparent.