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.