Previous cousin statistics charts haven’t shown the differences in shared DNA between paternal and maternal relatives.

The average recombination rate in mothers is about 42. Conversely, genomes in fathers only recombine about 27 times, on average. This leads to a conclusion that’s intuitive to geneticists: More recombination decreases variance, leading to narrower ranges in shared DNA for maternal relatives. Less recombination results in more variance, which is why fully or predominantly paternal relatives can share a much wider range of DNA. This phenomenon has been blogged about by Graham Coop.

You can judge the accuracy of a shared DNA chart or table by the known standard deviations of some of its data points.

On this page you’ll find standard deviations and ranges of shared DNA for 3/4 siblings and double 1st cousins that you can’t find anywhere else. Other popular sources even have the averages listed wrong for those relationship types. Elsewhere on this site you can find statistics for other complicated types of relationships, including pedigree collapse or the combination of multiple DNA kits to reproduce some of your ancestors’ DNA, neither of which are available anywhere else.

Data shown below are reported in percentages, which are more universal across genotyping platforms. If you want to see cM ranges for a particular site, please click one of the links below:



Table 1. Shared DNA between siblings. Standard deviations for relatives for which values are available in the literature to compare to are given one extra decimal point here to show how closely they approximate known values.

It’s hard to say which is a bigger advantage for this method of computing shared DNA averages and ranges, that it’s the most accurate method or that it can compute any combination of relatives. The latter function is illustrated below, as the model easily computes any type of three-quarter sibling (3/4 sibling) or double first cousin.

Table 2. Shared DNA between six different types of 3/4 siblings. (Three-quarter siblings.) HIR = ‘half-identical regions,’ where one of the two chromosome homologues matches. FIR = ‘fully-identical regions,’ where both copies of a chromosome match. HIR + FIR = all of the points on chromosomes where two people match once plus all of the points where they match on both copies. HIR counting includes FIR bp, but only counts them as if they’re half-identical.

Table 3. Shared DNA between double first cousins. All parameters are the same as for Tables 1-2.

Table 4. Shared DNA for grandparents and some of their descendants. All parameters are the same as for Tables 1-3.

Table 5. Shared DNA for descendants of grandparents, continued, for half-relationships. All parameters are the same as for Tables 1-4.

Table 6. Shared DNA for great-grandparents and some of their descendants.

Table 7. Shared DNA for second cousins. I was surprised to see that some second cousins may not share any DNA.

Table 8. Shared DNA for 2nd great-grandparents.

Table 9. Shared DNA for 3rd great-grandparents.

I hope you’ve found these results useful. More will be on the way.

Feel free to ask a question or leave a comment. And make sure to check out these ranges of shared centiMorgans, which are the only published values that match peer-reviewed standard deviations. Or, try a calculator that lets you find the amount of an ancestor’s DNA you have when combining multiple kits. I also have some older articles that are only on Medium.