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:
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?