Probability

[Bengt]

I don’t expect a definitive answer... but how likely is it that the autosomal test wouldn’t show any match whatsoever between the two individuals marked with yellow?

ECBD16A9-EC4B-4952-B334-81206D5FE38C.jpeg

[KATM]

The relationship you show in your diagram is 2nd cousins, once removed (2C1R). The woman in the left column is the great-grandchild of "Parent," and the man in the last box on the right is the great-great-grandchild of "Parent." You can use one of the many relationship charts found online, or the calculator at Ancestor Search to determine how two people are related.

Roberta Estes has a post on her blog, "Why Don’t I Match My Cousin?" According to the chart in that post, 2nd cousins (not removed) should virtually always share some DNA (>99% likelihood). 3rd cousins will have a >90% chance of sharing DNA, and a <10% chance of not sharing DNA. She advises seeing how the cousins match at GEDmatch (they can download their raw data files from FTDNA, register at GEDmatch, and upload the FTDNA raw data files). You can lower the segment size for matching at GEDmatch, so you might see more there.

I suppose a 2nd cousin once removed, being another generation removed, might have a very small chance to not share DNA; perhaps a bit larger than the up to 1% of a 2nd cousin. According to the DNA Detectives Autosomal Statistics Chart, 2C1R would generally share an average of 112 cM (1.56%), or a range from 30-215 cM (0.42%-3%). That chart is generally narrower in its ranges than some other estimates, per the article linked for it.

Have any of the other relatives of the two people in yellow tested? If so, do their relationships match as expected? You can use the Shared cM Project 3.0 tool at the DNA Painter website, for relationships which do show shared DNA.

[jimbirk]

There is a very low probability that 2C1R do not share DNA.

Refer to "Analyzing a Lack of Sharing in 2C1R Relationship"

https://thegeneticgenealogist.com/20...-relationship/

As the probability is very low make sure the relationship between the two cousins really is 2C1R.

[Bengt]

Many thanks for input and links. Very helpful!