Back earlier this year, a genealogist posted on one of the many genetic genealogy forums concerning the unknown relationship between two men. They shared in the range typically for second cousins; however, one man’s ancestry was known, and the other man’s lineage was not. Additionally, the two men did not share Y-DNA or mtDNA haplogroups.
The genealogist argued, since they did not share haplogroups, these two lines could be eliminated. I countered with the following argument: while that would generally be the case, it is not always that cut and dry. I additionally provided an illustration where a male (X14) did not share a Y-DNA haplogroup with five other males. My conclusion was that X14 was related through the patrilineal line of these five other men.
My Illustration
The five known related males were either tested or had the presumed haplotype of I1/I-M253 with the supposition of having the specific haplotype of I-A10921, as the father of four of the men had tested with the Big-Y700 test at FTDNA.
The remaining male, a first cousin to these four brothers tested with a 111-marker test at FTDNA and he matches two of his uncles with a 0 Genetic Distance at this level. The other uncle took the Big Y700 test and has the terminal haplogroup of I-A10921. This is the same terminal haplogroup that is shared with his brother, a second cousin, and two fourth cousins.
Our mystery male who tested at Y-37, however, has the predicted Y-DNA haplogroup of I2a1a, which is also known as I-P37. The writer of the initial post countered that I1 and I2 could be confused. According to YFull, I1 and I2 split from haplogroup “I” 27,500 years before the present. I don’t think there could be any confusion between I1 and I2.
In our analysis, we’ve added two females for an additional autosomal comparison. The following chart indicates the relationships among these seven relatives.
- Half Second Great Aunt/Uncle
- Second Cousin
- Half First Cousin, Once Removed or
- Half Second Great Niece/Nephew
The probability percentages for these relationships are found in the “Prediction” column. We’ll discuss the “Actual” column later. The ages of the participants would rule out the second great aunt/uncle and the second great niece/nephew predictions – leaving us with two relationship possibilities: second cousin or half first cousin, once removed.
The Relationship
These two relationships, however, are incorrect – but viable according to the predictions. The actual relationship is that of first cousin, once removed. In five of the predictions, the actual relationship shows as the second highest in all participants except with C09 and C13. In these cases, it is predicted as the third highest in probability. According to the Shared cM Project, the mean value for a first cousin, once removed is 433 cM with the largest represented group at this relationship is found between 401 cM and 500 cM.
To be honest, I have left out a few details, as this is a known relationship to this author. X14 and the seven comparison relatives share the same birth surname. He is the grandson of C01 – the brother of C02 and C03. X14’s mother (C-C) is the daughter of C01.
While it does not affect this analysis, C-N and C-K also share 20.3 cM on the X-chromosome with X14. C01’s X-DNA with both of his brothers is considerably lower than average for a full sibling. X14 shares the same amount of X-DNA with these two sisters and their father as does his mother and grandfather.
Low cM Numbers
As with any relationship, the amount of shared autosomal DNA can vary. According to the Shared cM Project, a first cousin, once removed relationship has a range of 102 cM to 980 cM. In this case, there are some anomalies present in this family. One is the shared cM between C01 and his full brother C02, which is 2398.2 or 41% by 23andMe’s calculations. Additionally, C01 and his grandson X14 share 1378.4 cM or 19%. These low numbers impact how much downstream relatives will share.
Conclusion
While the relationship is not through X14’s patrilineal lineage, it is through the patrilineal lineage of the seven comparison relatives. When looking at known relatives, do not discount the possibility that there may be a relationship through their match’s patrilineal or matrilineal lines although they do not share haplogroups. There are situations in families where this can and does occur. While this is not the normal way we would ascertain a match, it is best not to leave any stone unturned.
