What is expected shared cM of cousins with only one shared grandparent?



Related to What is expected shared cM of double third cousins?, what would the expected range and average shared cM be? In this case, I'm seeing 64.0 cM over 3 segments. Looking at the table, that is less than half the expected average (433 cM) and nearly outside half the range (115 cM to 753 cM). In this case, the further (1R) path includes a terminal paternal link that is a undocumented father with family stories giving the link.

Does this cast doubt on the half 1C1R and perhaps suggest it is more likely 2C1R (and no longer halved), that is the father of the illegitimate child was not the other's grandfather but instead his brother? In such a path, the cousins most recent shared ancestors are now a married couple, hence no more need to halve the relationship.

Chart of expected shared cM for various levels of cousins Source: ISOGG Autosomal DNA Statistics


Posted 2017-01-28T17:05:36.347

Reputation: 5 145



The original source of the data in the chart you have provided is The Shared cM Project. On their website, they provide more details in a PDF about the ranges provided, as well as histogram distributions of shared cM. The most recent update is from 25 Jun 2016. For some relationships, they do not have enough data points to produce histograms, but the table of those relationships gives you a rough idea:

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According to their data, half first cousins once removed share average of 187 cM (range 78-253 cM). This is based on 32 samples. This suggests that for this relationship one would expect you to share more DNA than you do (64 cM). However, given the small sample size you may just be one of the outliers. A 64 cM match could be anything from a 2nd cousin to a 4th cousin, with a number of removeds as well.

The amount of shared autosomal DNA is great for confirming paper-trail matches, but not so great for guessing a relationship with no documentation. Except for very close genetic relationships, there are simply so many different possible ways that one could get to a given shared cM. For such cases, especially those involving illegitimacy, one really needs to test and compare other relatives to triangulate, or utilise mtDNA or YDNA if possible.

Harry Vervet

Posted 2017-01-28T17:05:36.347

Reputation: 17 763