cMs shared

[lisalisa]

Hi,

my mom and I have both taken the dna test with ancestry, which is proving interesting in comparing amounts of shared dna down certain lines.

For example, I've noticed that on one particular line, I have approx 50% of the shared cMs that my mom has, so with someone on that line, mom shares 136 cMs across 7, I share 63 cMs across 4.  So that is pretty much what would be expected.
On another line, I have only approx 25% of the shared cMs of my mom's result, so I'd say I seem to have inherited less down that line.  Again that is understandable.

However what is puzzling me somewhat are the instances where I share more dna with the other person than my mom.   For example a new match today, my mom shares 9.6cM across 1, yet I share 13.2 across 1.
It isn't a case where my maternal and paternal lines are connected way back.
So as the dna I have from my maternal side can only be from the dna from my mother's dna, is this happening due to something connected with the statistical/mathematical analysis of the sample?

Anyone else experienced similar?

Lisa

[davidft]

Two articles that explain it

http://whoareyoumadeof.com/blog/2017/11/15/how-can-i-share-more-dna-with-a-match-than-my-mom-or-dad/

https://www.wikitree.com/g2g/466978/why-would-my-sons-centimorgan-count-be-more-than-mine (answer by Ken Sargent). I actually think this is the better answer if you read it a few times.


In short this is why its necessary to confirm matches by one to one matching and triangulation. Although as you have tested with ancestry I thought they did not give a chromosome breakdaown, unless of course you can access this by downloading the raw data to sites with chromosome breakdowns eg Gedcom, MyHeritage, FTDNA etc

[lisalisa]

thanks davidft,

I shall be reading these articles again (and probably again).
I haven't worked out the connection to the new match that came up today, I'll have to see if I've solved any of these similar situations only I know I've noticed a few going along.

Lisa

[VeeBeeFamilyTree]

Hi Lisa,
Glad you posted this as I had a similar query. My dad and I both have ancestry dna tests done. If I click on shared matches a list of people come up, clearly showing those who are related to me via my Dad’s dna. If I click on one person on that list I can also see shared dna with other members. Again surely all related by my dad’s side. Except on a fair few of them they don’t match with my dad at all! This shouldn’t be the case since the dna I share with them came from my dad and if anything my dad should be a closer match.  I am also fairly sure my parents are not at all related given they were born in different countries and there is no overlap in their family trees as far back as I can go.
Will take a good look at the links the other poster shared-much appreciated!

[melba_schmelba]

Hi Lisa,
Glad you posted this as I had a similar query. My dad and I both have ancestry dna tests done. If I click on shared matches a list of people come up, clearly showing those who are related to me via my Dad’s dna. If I click on one person on that list I can also see shared dna with other members. Again surely all related by my dad’s side. Except on a fair few of them they don’t match with my dad at all! This shouldn’t be the case since the dna I share with them came from my dad and if anything my dad should be a closer match.  I am also fairly sure my parents are not at all related given they were born in different countries and there is no overlap in their family trees as far back as I can go.
Will take a good look at the links the other poster shared-much appreciated!

I am wondering if this could be due to time since the test has been done, and the data hasn't fully gone into the match system yet? I know for the trees I added to my accounts, for one, it took me weeks to get any shared ancestor hints, and on the other, I only got one after about 6 months, when I had already found people whose trees matched by browsing manually shortly after I did the test - I am sure I am still missing many shared ancestor hints.

[VeeBeeFamilyTree]

I took a look back at the ones that shared match to me but not my dad. By clicking on their names you can see if they come up as a match for the two tests of my dad and me.
                                       The people in question are in fact distant cousin type matches for my dad. I believe the system only does 4th cousin or closer type matches on the shared matches list, hence why my dad doesn’t show up on the shared match list between me and the cousin.

On two of them I share more centimorgans of dna (about 1 or 2 cms) than my dad does. This tips them over the edge to qualify as 4-6th cousin rather than distant cousin matches. It seems that sometimes there can be dna data either side of the actual inherited dna that by coincidence matches these people. Possibly from dna from my mum maybe that happens to be the same by coincidence, at least that’s what the two articles above say.

Very confusing but clearly we need to be careful about fully relying on the ancestry matches. It’s a shame that more people haven’t got decent family trees uploaded as that’s really the useful bit. The ethnicity bit is fun but I don’t take it as gospel.

[melba_schmelba]

I took a look back at the ones that shared match to me but not my dad. By clicking on their names you can see if they come up as a match for the two tests of my dad and me.
                                       The people in question are in fact distant cousin type matches for my dad. I believe the system only does 4th cousin or closer type matches on the shared matches list, hence why my dad doesn’t show up on the shared match list between me and the cousin.

The cutoff point is 20 centimorgans (in total) for shared match lists on Ancestry. So if your Dad matches a person at 19.9 and you match at 20.1 you will show on the list and he won't. As far as I can understand, the way the DNA is processed is not 100% accurate and involves numerous algorithms etc. so there will be some funny anomalies. GEDMATCH uses different ways of analysing the data and they show completely different centimorgan amounts for the same kits that I match on ancestry. I also think (and remember being stated by an expert) that when you get to these small centimorgan amounts i.e. 6-20 it is not uncommon for them to be passed unchanged for generation after generation rather than getting much smaller as you'd expect with the big centimorgan matches.

[lisalisa]

I took a look back at the ones that shared match to me but not my dad. By clicking on their names you can see if they come up as a match for the two tests of my dad and me.
                                       The people in question are in fact distant cousin type matches for my dad. I believe the system only does 4th cousin or closer type matches on the shared matches list, hence why my dad doesn’t show up on the shared match list between me and the cousin.

I was going to ask if you'd looked at the profile page of the dna match person, as if you have 2 tests you can view/or manage, you can see both there (going between which of 'your' tests you are viewing) and that catches the cMs that are below 20 for the distant matches.  It is also handy for showing if the shared match person manages any other tests, and then you can see which of those you match with.  It's all nicely accessible on that profile page.

On two of them I share more centimorgans of dna (about 1 or 2 cms) than my dad does. This tips them over the edge to qualify as 4-6th cousin rather than distant cousin matches. It seems that sometimes there can be dna data either side of the actual inherited dna that by coincidence matches these people. Possibly from dna from my mum maybe that happens to be the same by coincidence, at least that’s what the two articles above say.

Very confusing but clearly we need to be careful about fully relying on the ancestry matches. It’s a shame that more people haven’t got decent family trees uploaded as that’s really the useful bit. The ethnicity bit is fun but I don’t take it as gospel.

I'm finding it fascinating and addictive even though I may have to think things through a few times to grasp them.   

Lisa

[VeeBeeFamilyTree]

Thanks to you both,

I am pretty new to the dna stuff and find it all interesting. It has helped with my family tree and verifying things I thought were correct but wasn’t 100% sure on.

Melba - May I ask what Gedmatch is? I have seen it mentioned on here and it sounds as if I can upload ancestry raw data to it? Is that correct?  Thanks