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According to Gedmatch's Are your parents related? tool, my grandmother's DNA shows several runs of homozygosity (ROH):

Chr Start Location  End Location    Centimorgans (cM)   SNPs
3   180690286       192749897       23.5                2151
8   11855773        21377758        17                  2555
9   22131034        30815690        11.3                1612
10  36514828        63439845        16.6                3005
14  93668370        106176088       28.4                2381

Gedmatch gives the conclusion:

This analysis indicates that your parents are probably related within the last several generations.

My grandmother's parents were known to be second cousins, so this is no surprise.

However, I am wondering how this data, in particular the number and length of ROH can help determine how closely one's parents are related.

This is my grandmother's family tree as determined via traditional genealogical methods, showing her inter-related ancestors:

enter image description here

As you can see, her parents were not only second cousins, but also double third cousins.

I have no doubt that the two great grandmothers were sisters, but proving the generation above (i.e. the fact that the two great great grandmothers were also sisters) has been challenging with paper records alone. Does this ROH data provide evidence in support of this genealogy? Or in other words, how does one interpret the number and length of ROH in terms of degree of relationship between the parents?

  • Harry rightly pointed out a flaw in the logic of an answer I had posted earlier today. Since I don't at the moment know how to address the issue raised, I've deleted that posting. – RobertShaw Nov 12 '18 at 7:39

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