Checking fully-identical regions (FIR, or IBD2) is by far the best way to distinguish between 3/4 siblings, full-siblings, and double 1st cousins (2x1C); identical-by-descent (IBD) is next best, and half-identical regions (HIR) is least preferable; IBD = HIR + FIR; cM = centiMorgan; 1C1R = 1st cousin, once removed.
This tool is only for comparison of 3/4 siblings or 2x1C to siblings and other close relationship types. If you don't suspect either of those less common relationships, or if you want to learn more about this tool, please click here.
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.
Averages and ranges for multiple cousins and other relationships 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.
All of the probabilities here are unweighted by proportions in the population. While population weight is a great idea for distant cousins, the DNA tester and their family have a better idea of the population likelihoods for close relatives. Showing unweighted probabilities here is best, so that individuals can then take into account their own population likelihoods. For each curve shown in the figure at the bottom of the page, 500,000 pairs were simulated. 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.
These probabilities are only calculated as far back as 1C1R since 3/4 siblings and 2x1C values won't be any lower than those. The huge advantage of this tool, other than the accuracy of the data and the inclusion of multiple cousins, is that it treats close relatives as not being in the same group because the curves are significantly different. 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.
Totals will not always add up to 100%. When more than one relationship type is possible, 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.