Three years ago my husband Greg and I sent off our DNA for analysis. There were three family history puzzles I thought DNA techniques might solve. One concerned the parents of Greg’s great grandfather Henry Sullivan. Who were they? I didn’t know. I thought DNA data might help.
We knew Henry was brought up in an orphanage in Geelong, but its records told us only that he had been abandoned by his parents, nothing more. We could not find a likely birth certificate for him.
I revisited the problem, reviewing digitised newspapers at Trove and widening my date search slightly. I wrote up the results at `Poor little chap‘.
Henry was committed as a State Ward on 11 June 1866. He was said to be four years old. Both his parents were living but he had been deserted. Looking at the newspapers for the month before Henry was committed I found a report that mentioned a ‘‘little one’ who had been abandoned by a woman named Sullivan.’ The newspaper stated that ‘The decrepit and indiscreet creature walked off with the child clinging to her.’. Perhaps this child was our Henry.
Following through various newspaper reports I came to the view that this Henry was the child of William Sullivan (born 1839) and Matilda Frances Sullivan formerly Hughes (born 1845). Matilda Sullivan was the daughter of Matilda Priscilla Hughes nee Moggridge formerly Derby (1825 – 1868) and the step-daughter of David Hughes (1822 – 1895). Matilda Sullivan had another son, Eleazer Hughes (1861 – 1949). Eleazer Hughes had left descendants. I hoped that by matching Greg’s DNA with the descendants of Eleazer Hughes I might be able to confirm the hypothesis of Henry Sullivan’s parents.
Greg and his brother Dennis, his first cousins BS and MS, and his second cousin LB all share DNA with various descendants of Eleazer Hughes.
The challenge with DNA matching is to be confident about which of your forebears you have inherited the shared DNA from.
For close relations where you knew the test-takers beforehand and when the amount of shared DNA corresponds to the amount expected to be shared given the relationship, a shared DNA match is taken to be evidence of the relationship. If you have access to the shared chromosome details then you can attribute the shared ancestry to the shared DNA.
When the relationship is more distant you need to be confident that the DNA is shared from a particular ancestor and not from some other shared ancestor. That other ancestor may be on a part of the tree you or your match have not yet documented, that is, you do not know about your shared relationship. A measure of this is tree completeness -how many of your forebears have you documented for the necessary generations. If you are looking at an expected third cousin relationship then you expect to share great great grandparents. The question then becomes whether you and your match have both documented all sixteen of your great great grandparents. Only then can you be completely confident there is no another possible explanation of why you share DNA.
When it comes to fourth cousin relationships you are one more generation back. Both you and your match need to have documented thirty-two third great grandparents but also you need to take into consideration other possible relationships that might account for the amount of DNA that you share.
The distance between two gene loci on a chromosome is measured in centiMorgans (cM), defined as ‘the distance between chromosome positions for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01’, that is, how likely the segment is to recombine as it passes from parent to child.
If two sets of DNA are compared, a higher number of shared centiMorgans means greater confidence in the match, that is, greater confidence that the match represents a closer relationship.
Any given number of centiMorgans though can represent a variety of relationships. The Shared cM Project is a collaborative data collection and analysis project created as part of research into the ranges of shared centiMorgans associated with various known relationships. A tool called the ‘Shared cM Project 3.0 tool’ v4 allows users to compare the amount of DNA shared with a match with the accumulated results of the data collection of more than 25,000 relationships and their shared DNA. Using the tool is an aid to understanding what relationships are most likely to be represented by the amount of shared DNA.
The more generations back the higher the chance that no DNA is shared between descendants. It is possible for third cousins not to share DNA and the likelihood that fourth cousins share DNA is only in the order of 50%.
If three people share one segment of DNA and they know how they are related, then we have more confidence that the shared DNA comes from particular ancestors.
Greg and L B are second cousins. They have tested their DNA at AncestryDNA and uploaded to MyHeritage and GedMatch. At AncestryDNA they share 242 centiMorgans across 9 segments and at MyHeritage they share 254.6 centiMorgans across 9 segments. (I have previously discussed my experience of variations in DNA matches between
Greg and L B are half third cousins to D J G. D J G’s great grandfather, Eleazer Hughes, was the half-brother of Greg and L B’s great grandfather Henry Sullivan. At MyHeritage Greg shares 89.2 centiMorgans across 4 segments with D J G. L B shares 64.1 centiMorgans with D J G. The amount of DNA shared between the cousins falls within the probabilities predicted using the shared cM tool.
Greg, L B and D J G share one triangulated segment on chromosome 10. The segment is 47.7 centiMorgans long.
I believe this DNA segment on chromosome 10 was inherited from Matilda by Greg, L B and D J G.
I checked that there was no other likely relationship to explain the DNA match by tracing the grandparents of DJG. Greg’s family tree and the tree of LB are both complete and documented up to their great great grandparents.
LB and Greg do have other matches with descendants of Eleazer Hughes but so far I have not been able to triangulate the DNA to a single segment. AncestryDNA, which has the most DNA matches, unfortunately lacks the tool, a chromosome browser, to demonstrate the triangulation.
Postscript: the poor little chap grew up, married and had a family. It seems he had a contended adult life. You can read about him at H is for Henry.