Double cousin predictions now incorporate the number of segments for much more accuracy
Predictions from DNA-Sci, which are the only ones that come from a peer-reviewed data source, now employ the number of segments and many more double cousin relationships. You can find the new double cousin predictor here. This is the third double cousin predictor ever created, all of which are hosted by DNA-Sci.
Types of Double Cousins (newly included types are highlighted)
The following double cousin relationship types are now included in predictions:
- 3/4 Siblings: Child of your father and maternal aunt
- 3/4 Siblings: Child of your mother and paternal uncle
- 3/4 Siblings: Child of your father and maternal half-sister
- 3/4 Siblings: Child of your father and maternal grandmother (opposite perspective of the immediately above type)
- 3/4 Siblings: Child of your mother and paternal half-brother
- 3/4 Siblings: Child of your mother and paternal grandfather (opposite perspective of the immediately above type)
- 5/8 Siblings: Half-sibling + half 1st cousin, i.e. a child of your parent and the other’s parent’s half-sibling
- Double First Cousins (2x 1C): “Two brothers married two sisters” or “A brother and a sister married a sister and a brother” is how it’s often said
- 1C + Half- Aunt/Uncle/Niece/Nephew
- Child of Your 3/4 Sibling (1C1R + Half- Aunt/Uncle/Niece/Nephew): A child of any of the types of 3/4 sibling above; or your parent’s 3/4 sibling
- 1C + First cousin once removed (1C1R)
- 1C + Half-1C
- Half- Aunt/Uncle/Niece/Nephew + Half-1C
- Half- Aunt/Uncle/Niece/Nephew + 1C1R
- Double 1C1R (with or without FIR)
- Children of 3/4 Siblings (Half-1C + 2C): Your parent is a 3/4 sibling of this person’s parent
- Double Second Cousins (with or without FIR)
- Double 2C1R
- Double Third cousins
- Double 3C1R
- Double Fourth Cousins
The term 5/8 sibling was coined by Richard Handley. He first published the term in the October 2022 issue of The Irish Tree, the newsletter of the Irish Genealogical Society of Michigan.
Do you have a suggestion for a type of double or multiple cousin relationship to add to this tool? If so, please leave a comment below.
If you’d like to submit double cousin data or any other DNA matching data in order to improve tools like this one, please submit them here.
Which types of double cousins will share FIR?
For many of the double cousin relationships, two types are included here. One type cannot share fully-identical regions (FIR) and the other type usually will. Including both was necessary because there are differences between the two in the half-identical region (HIR) amount, which is the only way cM values are reported at Ancestry. For HIR comparisons of relationships, the type that cannot include FIR will actually have a higher cM value. This is because a double 2nd cousin (2C) pair, for example, who cannot share FIR, will have an average shared amount of 6.25%, all of which comes from HIR. Conversely, a double 2C pair who do share FIR will usually only have about 6.05% HIR, the other 0.2% coming from FIR. Also, double cousin types with possible FIR will also have slightly lower amounts of full IBD sharing at genotyping sites. That’s because some segments that are FIR will be below the low-cM threshold, causing twice the amount of shared cM to be discarded for those segments when the cutoff is applied.
How do you know if your double relationship could include FIR? Here’s the test: If both of your parents are related to the match and you’re related to both of your match’s parents, then you could share FIR with your match. Please note that as double relationships get more distant, it becomes less likely that they’re aligned in exactly the right way to produce FIR.
Which checkboxes to use
As with the previous iteration, the 23andMe part of this tool includes options for three different measuring metrics, in order of the most useful predictions: total identical-by-descent sharing (IBD, like what’s reported at 23andMe), FIR (which can be seen at 23andMe or GEDmatch), or HIR (the default for all sites other than 23andMe). If you leave the default options for IBD, FIR, and HIR, those will be the right ones to use the vast majority of the time.
Other relationships included
Probabilities are included for other (non-double) relationships as far back as 14th cousins, but half 7th cousins and further back are now all included in a group called “distant cousins.” This tool does not include population weights, which are easier to implement with traditional relationships. Double cousin relationships are less common and there may not be a perfect way to add population weights to a relationship predictor that includes them.
For more information about the methodology and discoveries associated with this tool, click here.
The data used for these predictions came from Ped-sim. In this case, the refined genetic map of Bhérer et al. (2017) was used as well as the crossover interference parameters of Campbell et al. (2015).
All of the most advanced relationship predictors use the same peer-reviewed data source. Here are the tools of note:
DNA-Sci — advancing the science of relationship predictions. Please also submit data to this new DNA match survey that will greatly help improve and build new relationship prediction tools. Feel free to ask a question or leave a comment. You might also like this tool to visualize how much DNA full-siblings share. DNA-Sci is also the original home of DNA coverage calculations.
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