Most Probable Relationship Type
Avuncular relationships = aunt/uncle/niece/nephew; 1C1R = 1st cousin, once removed; cM = centiMorgan, HIR = half-identical regions; IBD = identical by descent (HIR + FIR).
All probabilities are for autosomal DNA only. Please subtract any X-DNA before using the calculator. Also, I recommend subtracting any shared DNA from segments less than 7 cM that may have found their way into your total.
The above probabilities assume no endogamy or other pedigree collapse. Those cases should be treated separately.
Multiple cousin relationships are not included here, but averages and ranges can be found here.
Parent/child relationships are not included here. They are easy to distinguish from other relationships, including full-siblings. Parent/child relationships consist of a half-identical match across the whole length of the genome. Full-siblings share 25% fully-identical regions (FIR), on average. Genotyping sites will take this into account in their relationship prediction. If a relationship is predicted to be parent/child, full-sibling is not a possible relationship and there is no need to analyze the shared DNA amount here.
Relationships more distant than 1C1R and half-1C are grouped together by those with the same average shared DNA. Also, half-avuncular relationships are treated the same as siblings of grandparents, which are called great- or grad-avuncular relationships. They are treated the same because the curves are the same, as are any other relationship types that share the same curve. For each curve shown in the figure at the bottom of the page, 500,000 pairs were simulated. Therefore, relative probabilities of each relationship type are based on the assumption that an equal number of each are possible in the population. While this assumption isn't true, it's the best way to generate probabilities. Age and other factors, such as the likelihood that your unknown great-grandparent or great-grandchild is the DNA match you've found, should be taken into consideration. It's probably more likely that a 1,200 cM match is a half-avuncular relationship than a great-grandparent, despite the fact that, if they were equally likely relatives to find as DNA matches, the cM value alone suggests great-grandparent is more likely.
These probabilities are only calculated as far back as 5C1R. The huge advantage of this tool, other than the accuracy of the data, is that it treats close relatives as not being in the same group because the curves are significantly different. For distant relatives, there's much less certainty about the genealogical relationship for your DNA matches. Matches as low as 8 cM are allowed here, however the relationship may be farther back than 5C1R. However, the relative probabilities may be accurate even at those low values. Indeed, any of the probabilities shown above are only relative to the other relationships listed, therefore they’re only meaningful in comparison to the other relationships. And there's no cM value at 8 cM or above at which even a 4C1R is the most probable relationship. So, while the probability of an 8 cM match may be higher for "4C1R or more distant," listing each relationship type separately would not result in more useful information. Not only are very low cM values difficult to assign to a recent ancestor, but segments of 20 cM or 30 cM may be on pile-up regions and therefore come from very distant ancestors.
Totals will not always add up to 100%. When multiple relationship types are present, the chances of rounding errors increases. I don’t believe that the totals are ever off by more than 0.2 percentage points. For more information about the methodology and discoveries associated with this tool, click here.
This is not the first tool to show relationship probabilities based on a user input of shared DNA. Jonny Perl has done amazing work at DNA Painter, including probability calculations that can be built-in to your family tree, and Genetic Affairs also displays relationship probabilities.