Any ideas what this means?

[kiterunner]

Going through my DNA matches, I am finding that there are quite a lot of matches with whom I share about 16 cM in 1 segment, and who are also matches to my Dad, but sharing only 7.7 cM with him (also in 1 segment). None of them has shared matches so far, though some of them are certainly closely related to each other, but of course Ancestry only gives shared matches which are above 20 cM.

Do you think I can assume that they all share the same segment with me, and the same segment with my Dad, and that they are all therefore related to us on the same line, if I ever figure out what that is? I'm imagining that all those people inherited the 16 cM segment from our common ancestor, and my Dad inherited the 7.7 cM and another small segment (below ancestry's cut-off, think it is 5 cM?) with a bit of DNA from another ancestor in between them, and that in recombination, I got a bit of DNA which happened to match the bit which the common ancestor had in the middle, thus making it back up to 16 cM. Or is that not how it works?

I'm hoping I can find some of these matches on My Heritage to see if it is the same chromosome in all cases, but haven't found any on there yet. Of course they must be pretty distantly related to me.

[Kit]

Is there any chance your parents share some ancestors and you got more from your Mum than your Dad?

Otherwise I have no idea. Your assumption sounds ok to me but I have no idea if science backs it up.

[kiterunner]

Thanks for the suggestion, Toni, but I don't think that it would say 1 segment if it was over 2 chromosomes (one from my Mum, one from my Dad), though, would it?

[Kit]

I didn't know a segment meant a chromosome so when you put it that way no, it is very unlikely.

I have heard of chromosomes doing odd things ie splitting and rejoining but not in regards to genealogy and I have no idea how that would show up. That situation is also rare.

I have really avoided the whole science of this dna testing. I know the higher the number, the stronger the relationship but the segments I haven't really understood.

[Phoenix]

I have a DNA match 6.1Cm over two segments. A segment must be under 3.05Cm.

Does Ancestry define "segment" anywhere?
Do they include crumbs?

Does Ancestry ever make mistakes??

Clearly, you could inherit 5.9 from your mother. That still leaves a large chunk unaccounted for.

[kiterunner]

I'm thinking now that maybe the 7.7 cM is in the middle of the 16 cM block and there are two small gaps either side of it that have been filled in with DNA from the other matching chromosome.

[Kit]

I'm not sure how chromosomes work exactly but we get half from each of our parents. Normally A1 matches with A2, B1 with B2, down the line until the sex chromosomes which are different.

Sometimes however things don't work and A1 for example, breaks. If A1 loses a part there can be some sort of medical condition or disability, I know with a type of epilepsy one sydnrome is a deletion on a chromosome. In other situations A1 breaks but somehow part of A1 merges with D1 and part of D1 merges with the other part of A1, leaving 2 whole chromosomes and there is no issue and you would only know through genetic testing.

I found this on 23andme:

If you are female, you share 23 segments with each parent, and each segment spans the entire chromosome. If you are male, you share 22 segments with father (since the DNA Relatives feature does not use the Y chromosome) and 23 segments with your mother.

Given that I don't understand how you have a stronger match than your father.

[Phoenix]

I'm not sure how chromosomes work exactly but we get half from each of our parents. Normally A1 matches with A2, B1 with B2, down the line until the sex chromosomes which are different.

Sometimes however things don't work and A1 for example, breaks. If A1 loses a part there can be some sort of medical condition or disability, I know with a type of epilepsy one sydnrome is a deletion on a chromosome. In other situations A1 breaks but somehow part of A1 merges with D1 and part of D1 merges with the other part of A1, leaving 2 whole chromosomes and there is no issue and you would only know through genetic testing.

I found this on 23andme:

If you are female, you share 23 segments with each parent, and each segment spans the entire chromosome. If you are male, you share 22 segments with father (since the DNA Relatives feature does not use the Y chromosome) and 23 segments with your mother.

Given that I don't understand how you have a stronger match than your father.


Hmm. I have 60 segments matched with sib's child, and 40 matched with cousin.

[kiterunner]

I didn't know a segment meant a chromosome so when you put it that way no, it is very unlikely.

I have heard of chromosomes doing odd things ie splitting and rejoining but not in regards to genealogy and I have no idea how that would show up. That situation is also rare.

I have really avoided the whole science of this dna testing. I know the higher the number, the stronger the relationship but the segments I haven't really understood.
No, a segment doesn't mean a chromosome, but I don't think it would say 1 segment if it was split over the 2 matching chromosomes.

[Phoenix]

I found this article, but it is as clear as mud to me, and I don't know whether the same algorithms are used today:

https://blogs.ancestry.com/ancestry/2016/01/06/behind-the-new-ancestrydna-feature-amount-of-shared-dna/

[kiterunner]

OK, ancestry says I share 3,421 cM with my Dad across 81 segments, while My Heritage says 3,534 cM across 22 segments. So in this instance, MyHeritage is counting each full chromosome as a segment, because all the DNA on 1 of each pair of my chromosomes comes from one or other of Dad's corresponding chromosomes. It doesn't split the chromosomes into segments based on which of his 2 chromosomes any particular bit of DNA came from. As for ancestry, I know they filter out some of the common Jewish DNA, so I don't know whether that is why they say 81 segments, or if they do actually only count as a segment if it is all on the same one of his chromosomes. I can't find an example of a typical parent / child comparison on Ancestry at the moment to see whether they usually say 22 segments or not.

[kiterunner]

Ooh, thanks for that link, Phoenix! I guess you answered my question on post #11 before I had even asked it!

Still not sure what the situation is with my matches but hopefully I will find out eventually, and will be sure to report back when I do!

[kiterunner]

Bother, I wish I had looked at some of these while the extra shared matches were showing. Maybe it would have given me a clue. I will try to remember to do it if they show them again.

[kiterunner]

Now we have the "longest segment" info available, I have looked at one of the DNA matches that I was asking about, and it says "longest segment 19 cM" for both me and my Dad, so it is the Timber algorithm removing more of that segment from the match to my Dad than from the match to me. As we suspected.