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General => Ancestral Family Tree DNA Testing => Topic started by: AngelaR on Saturday 20 January 18 15:08 GMT (UK)
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I have been reluctant to post on this board because I have been struggling to understand the DNA test results ever since I had them from FamilyTreeDNA 4 or more years ago and am still very ignorant.
However, I have chased the people who are listed as 2nd-4th cousins and have found a few confirmed matches so am reasonably happy.
BUT - one of my 2nd-4th matches is marked on the FamilyFinder results as also matching me on an X-chromosome. Since I know(I think) that I get my X-chromosomes from each parent, does this finding have any significance at all? I understand, in a very simplistic way, that the longer the bit of shared centimorgans the better the match but the chromosome information defeats me.
If someone could give me a hint, it would be much appreciated! I assume that FTDNA wouldn't include the information if it wasn't significant but perhaps I'm naive here?
Many thanks for your patience with this....
Angela
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Hi Angela,
This is just my understanding :-) First check the size of the x-match, if its a tiny match then I wouldn't consider it as that's probably just random, but if its a bigger match then you can use it to help narrow down where the connection may be. The way you can do this is that a male can only inherit the X gene from his mother, so if you have a reasonable size match you can eliminate any male ancestors of a male, which obviously removes a lot of potential branches.
Richard
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Yes, I agree with Richard. The X chromosome match is the same as any other match from the Family Finder test, except that a male must inherit their X-Chromosome from their mother, so that narrows down possibilities somewhat.
So as far as I know, the X match will be included in the assessment of your match as 2nd-4th cousin.
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Hi Richard and Eric
Thank you so very much for your helpful explanations. I braved looking at FTNA's chromosome browser, which wasn't very helpful, being too "broad brush" I think, so then downloaded the spreadsheet and looked at that.
The "match" person had 4 bits of matching sequence on the X chromosome, the biggest of which was 2.4 (centimorgans?). Would those classify as small enought to be random do you think? I rather hope they are because, judging by the other matches I have already traced, this person must be well back on my father's father's father's line which, as you say, wouldn't fit with matching x-chromosome.
I so much appreciate your help with this as I have been struggling to get my head round it all for a long time and light may be starting to emerge.... :)
Thanks again
Angela
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Hi Angela, I am far from being an expert, but here is my understanding.
DNA is very variable, and two siblings may get quite different amounts of DNA and different segments from each parent, so all predicted relationships can only be approximate. The two main ways to predict a relationship are the total cM of matched segments and the size of the longest matching segment. Testing companies and other experts have slightly different ways to calculate the predicted relationship using those two numbers, but I think generally the total cM is more important than the cM of the largest segment. And I think there is some cutoff, and smaller segments than the cutoff are not counted.
A match of 2.4 cM is a pretty small segment, and could easily occur by chance, or, I guess really from a very distant common ancestor. I would guess that if FTDNA has this person as 2nd to 4th cousin, that the total cM in common is about 50-70 cM and therefore this segment has little impact on that predicted relationship. Most experienced genealogists take note of small segments, but seem to think that anything less than about 10 cM is not highly significant.
I hope that helps.
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In the Chromosome Browser you can chose whether to show matching blocks of 1, 5, 10 bits (I think there are actually four options). The coloured blocks show where that person matches to you. If you select a few people to see in the browser it can help to see if they are on the same branch of your family, if they all match you in the same largish blocks then you probably all have a common ancestor (or two).
Mary
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Angela, I am glad you asked this question as I have been in the same position as you for a similar length of time. :)
I have a "good" match with someone and also match on the X chromosome. I am female (so is she) and I am 100% certain this match has to be on my maternal line.
I can't find any common names at all.
I have no idea how this works or if it is right or if I have completely misunderstood what I have read (and it was some time ago that I read it so I am very likely to have both misunderstood and forgotten) .... but, I read somewhere that X matches relate to areas. :-\ I can see that some of my X matches ancestors and some of my ancestors did come from a similar area of the country.
Still I can't see a surname match.
It is all fairly mind boggling. :)
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I have just found some notes I wrote:
I have a DNA match with a second cousin once removed. I had made contact with her before I found her as one of my FTDNA matches. My great great grandparents and her great grandparents are the same people.
We have 109 shared centimorgans - longest block is 46.
My other contact, who I cannot find a connection with -: we share 99 centimorgans - longest block is 36 and we have an "X-Match."
I read that shared centimorgans relate to a geographical area - the longest block can be old and untraceable, and the DNA passes down randomly.
Apparently by comparing on the chromosome browser my known second cousin shares 16 segments, and my new contact shares 13 segments.
We have to be fairly closely related surely?
???
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Hi Ruskie,
Again, I'm far from an expert, but here's what I understand ....
If you and another person, both females, match on the X chromosome, it could be either male or female lines for either or both of you. It is only men who can know that an X-match means female line (because their other 23rd chromosome is Y). That means you might see some familiar surnames (if it is male line) or you might not (especially if it is female line).
I don't know about X matches being related to areas. I think in times gone by, people didn't move around so much as today, so any matches with common ancestors a few generations ago are likely to have been from the same area, but not certain.
To add to the surname difficulties, sometimes surnames changed or are unexpected - maybe the woman wasn't married and so the child may have been given her name, or the name of who she wanted to claim as father whether he was or not (I have researched a couple of cases like that), sometimes people changed their names and sometimes they were wrongly recorded. These are the sorts of difficulties where paper trails can lead to a brick wall, and DNA can help.
In the end, we need a little luck, a lot of persistence and some assistance from those we match with, to trace possible connections. Best of luck!
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If you match them both on the same segment(s) and that/those segments are reasonably large, and if the others also match on the same segment, then you almost certainly are related. The key is all matching on the same segments (which is called triangulation), which is very unlikely unless you are related.
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Hi Eric. Thanks for your thoughts on this.
It is (definitely) my maternal line which must match my contact's maternal or paternal lines.
What you say about regional matches makes sense. I do have some suspicions as to which of my family lines the match may come from. None of our surnames are the same but there could be a NPE somewhere along the line.
I will look to see which segments my two contacts match on, but if I am on the right rack, they won't match as they are different branches - my known second cousin is from my grandfather's maternal side, and the other I suspect may be on my grandfather's paternal side. :-\