Sim-cM: An Update

A couple of weeks ago, I introduced Sim-cM – a project aiming to provide data from simulated pedigrees for different relationships of genealogical interest.

I have now added more data, with results posted for 66 different relationships (including all of those listed in the Shared cM Project Version 3.0). For each relationship, I have included: percent of relatives that share (detectable) DNA, average amount of DNA shared, average number of segments shared, and average size of largest segment shared across 10,000 replicate simulations. Histograms and average values can be found at the Sim-cM page. I have also made raw data for all 10,000 replicates available online as text files, for anyone who wants to examine the data more closely or use it in other ways. For more detail about the methods used, see the Sim-cM White Paper.

I also found minor errors in some of the first results published in September, so these results (averages, plots, and raw data) have been updated.

As discussed in the white paper, Sim-cM results for many of the relationships show lower amounts of DNA shared on average than do the results for the same relationships in the Shared cM Project (Version 3.0). The plots below compare the averages per relationship for all relationships that appear in both datasets.

Average amounts of DNA shared per relationship from the Shared cM Project (in blue) compared with that from Sim-cM (yellow). Note that the two plots above are plotted on different scales since the relationships in the second plot share less DNA on average.

Data from Sim-cM also highlight other patterns. For example, when looking at numbers of segments shared, some distributions are bimodal – most notably those for half siblings and grandparents/grandchildren.

Previous observations have shown that DNA shared between half siblings and between grandparents and grandchildren can vary based on whether the relatives are connected by males or females. Since recombination rates have been shown to be lower in males than in females, half siblings who share a father will typically share fewer segments than those who share a mother.1

The above plots include all simulated data for the relationship, regardless of the sex of the individuals involved. The results shown, though, are consistent with an expectation of distinct distributions of number of shared segments with maternal vs. paternal inheritance.

For more detail on this topic, see:

1.  Madison Caballero, Daniel N. Seidman, Thomas D. Dyer, et al., “Crossover interference and sex-specific genetic maps shape identical by descent sharing in close relatives,” BioRxiv, 17 June 2019; preprint article ( : accessed 2 October 2019). 


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