Close relatives like two full siblings, an aunt and nephew, or a grandparent and grandchild always share IBD segments, so they show up in testing companies’ relative matches. However, more distant relatives may not share any IBD segments. In fact, the chance that two people share DNA decreases with the distance of their relationship. This is important to remember when doing genetic genealogy: if you don’t share segments with someone that doesn’t necessarily mean you’re not related to them. As the numbers below show, even some rare second cousins (0.02% based on this analysis) may not have any detected IBD segments.
To find out the rates that relatives share segments, one option is to simulate. We did this previously (Figure 3, the SS+intf bars), but that work counted all segments, regardless of their length. Unfortunately, reliably detecting segments shorter than 6-7 cM is hard and most companies only look for 6 or 7 cM or longer segments.
Considering only 7 cM or longer segments changes the rates that relatives share DNA, as shown in the plot below. The numbers above each bar give the percent of each relative type that share at least one ≥ 7 cM segment. (Here 1C represents first cousins, 2C second cousins, etc., and NC1R represents Nth cousins once removed.) From this, we see that first cousins share five or more ≥ 7 cM segments 100% of the time, while only 0.286% of eighth cousins share such a segment (and nearly all share only one). (See below for details on how we simulated.)
You can hover over the bars to see the percentage breakdowns across segment counts.
These numbers are from simulated relatives: 100,000 pairs for each type. If the segment is present, the simulator always reports it. A caveat therefore is that, while companies report many of the ≥ 7 cM segments, they sometimes miss some. (They also sometimes report a segment that is not real, unfortunately, though in most cases a ≥ 7 cM segment will be real.) Therefore, these numbers should be used as a guide. We could—and a future blog post may—update the numbers based on probabilities of detecting segments, but a challenge is that detection rates depend on many factors, including how many SNPs were tested in the two relatives and the method the companies use to detect the segments.
Other relative types
The simulations considered a range of full cousins and full cousins once removed. It turns out, a full Nth cousin has the same shared segment properties as a full (N-1)th cousin twice removed, so the sharing rates here apply to many more types of relatives. Specific examples of equivalent relatives are shown below along with general cases. (This table doesn’t list all relative types.)
|3C||2C2R, 1C4R||half-2C1R, half-1C3R|
|3C1R||2C3R, 1C5R||half-3C, half-2C2R, …|
|4C||3C2R, 2C4R, 1C6R||half-3C1R, half-2C3R, …|
|4C1R||3C3R, 2C5R, 1C7R||half-4C, half-3C2R, …|
|NC||(N-1)C2R, (N-2)C4R, (N-3)C6R, …||half-(N-1)C1R, half-(N-2)C3R, …|
|NC1R||(N-1)C3R, (N-2)C5R, (N-3)C7R, …||half-NC, half-NC2R, …|
Half relatives such as half-first cousins (who share one common grandparent instead of two as in full first cousins) have very slightly lower rates of sharing segments than full relatives of the roughly equivalent type.
If there’s enough interest (on Twitter or in the comments), we can put up another post on half-relatives. Update: See the next post for rates in half relatives.
The numbers in the plot above are based output from the Ped-sim program where we used a sex-specific genetic map and modeled crossover interference. We found that Ped-sim very accurately captures the total segment length that real relatives share, so the numbers in the plot should be very reliable in a scenario where a company detects all ≥ 7 cM segments with no false segments. You can run Ped-sim with sex-specific maps and interference here.
Thanks to Jonny Perl for asking about sharing rates of 4C2R, which helped motivate this post.
I would be very interested to see a chart or calculations where Endogamy is involved as when a match is a 2nd cousin, a 3rd cousin and a 3rd cousin (and/or variants). The total cM’s should be less than the sum of the 3 relationships, but which relationships cM’s would be reduced and by how much?
Wouldn’t be too hard to look. Note that if the relationships are through different parents, the total cM’s should be the sum. Of course, that’s only if you could “fully identical” regions — IBD2 in other contexts — as double the cMs.
In one WATO tree, I have double cousins (both (xG)-GPs married siblings). This yields double cM values, so reduces probabilities in the table at the end. WATO seems much happier when I halve the cMs for all these double cousins (& kids) and ups the probabilities since the cMs are more consistent with the relationship (ignoring the double cousins).
Nice post. Well done.
Like it – Jonny could put this into a look-up?
I am interested in half relatives. Happens a lot and is of course important for tracking down NPE relationships
Thanks! I’ll post this hopefully this weekend.
Thank you for this informative post. I would like to see a post on half relationships as well.
You got it! Check back late Saturday.
I would love to se a post about 1/2 relatives. After all, remarriage and half-sibs, are fairly common – and always was and then there are all the “unoffical” relations !
“A half cousin has a very slightly lower rate of sharing segments than full”, it would seem because they should actually share less cMs.
I have a 5th cousin relationship with many cousins who are descended from ancestral grandfather and wife #1; but since I descend from wife #2, I have to drop the threshold to find a lower cMs sharing, but at least I do match them.
I just subscribed to your blog and look forward to reading more.
Right. Half-Nth cousins are not (roughly) equivalent to Nth cousins, but to Nth cousins once removed. Stay tuned for the rates of half relatives soon!
I love the bar chart and explanation. Thank you for posting.
Very helpful explanation. Thanks.
I found your stacked bar chart showing DNA segment probabilities for cousin relationships very helpful. I am particularly interested in determining 5th cousins from my Revolutionary War Loyalist Ancestors that relocated to Ontario; with the objective of identifying my 4th Great Grandparents. My question is can a probability distribution of segments lengths be used to help discern distant cousin relationships? For example, I see some putative 5th cousin single shared segments over 50 cM up to 100 cM. Thanks.
Segment lengths usually say very little about how distant the cousins are. It is a bit odd though to have only one segment longer than 50 cM. Is it possible that there’s pedigree collapse or endogamy here?