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View Full Version : Ancestry Composition Segments And IBD Segments The Same?



J Man
07-18-2014, 10:49 PM
Are the segments that we we have on Ancestry Composition at 23andme that for example can be listed as ''British and Irish'' or ''Finnish'' the same as the IBD segments that we share with individuals on features like Relative Finder of Family Finder over at FTDNA?

AJL
07-18-2014, 11:09 PM
Yes and no: for AC they use short phased blocks.

https://www.23andme.com/ancestry_composition_guide/

J Man
07-18-2014, 11:40 PM
Yes and no: for AC they use short phased blocks.

https://www.23andme.com/ancestry_composition_guide/

Okay well for example I have seen some French people who score around 45% or so in the British and Irish category on Ancestry Composition. I doubt those people share long IBD (15 cM+) segments with many Irish and Brits though. What do you think?

AJL
07-18-2014, 11:52 PM
^ Yeah, that's somewhat likely to be overrepresentation of British in the database and a failure to adjust for that.

J Man
07-19-2014, 12:21 AM
^ Yeah, that's somewhat likely to be overrepresentation of British in the database and a failure to adjust for that.

I don;t really understand this very well. How can a person of French or mostly French ancestry score around say 45% in the British and Irish category on Ancestry Composition but not also share numerous large 15 cM and larger IBD segments with Brits and Irish?

AJL
07-19-2014, 12:51 AM
I don;t really understand this very well. How can a person of French or mostly French ancestry score around say 45% in the British and Irish category on Ancestry Composition but not also share numerous large 15 cM and larger IBD segments with Brits and Irish?

It's because even a phased block of 3 cM could be much older and is essentially not "worth the same" as an unphased block of 15.

J Man
07-26-2014, 12:06 AM
It's because even a phased block of 3 cM could be much older and is essentially not "worth the same" as an unphased block of 15.

So do you mean that all of those long blocks that French individuals have that show up as British and Irish on Ancestry Composition may just be small IBD segments around say 3 cM or so that are just stitched together? I am not quite sure what you mean sorry.

AJL
07-26-2014, 02:08 AM
So do you mean that all of those long blocks that French individuals have that show up as British and Irish on Ancestry Composition may just be small IBD segments around say 3 cM or so that are just stitched together? I am not quite sure what you mean sorry.

They might not even be IBD in any meaningful sense.

Dimanto
11-10-2014, 11:34 PM
They might not even be IBD in any meaningful sense.

How would you name it then ?

ArmandoR1b
11-10-2014, 11:58 PM
Ancestry-informative markers called AIMs for short

geebee
09-08-2017, 01:16 AM
I have found that it is sometimes quite possible to correlate Ancestry Composition segments and segments shared with relatives. Not always, by any means; but sometimes.

An example is a Native American segment that three of my siblings and I have on chromosome 15. (I also passed this segment on to my daughter.) I have a confirmed 3rd cousin who shares with me at this same location and he also shows a Native American segment in this location.

We have a common, documented Native American ancestor from several more generations back than our closest common ancestor, but both of us have multiple lines from that ancestor. At Ancestry -- which of course lacks any sort of chromosome browser -- we also have a number of other relatives who descend from this ancestor and have at least a trace amount of Native American (some have more, especially those with multiple connections).

All of my siblings have a similar total amount of Native American, which is fairly small -- around 2%. But it's consistent across multiple companies. My youngest sister is the only one of us who shows a Native American segment on the X chromosome. It's located in a region she doesn't share with the rest of us, but it is from our mother. I've traced my mother's ancestry back to the Native American ancestor following only X paths, and she has at least two of these.

Despite the protests of some to the contrary, it's clear that even relatively large segments can persist over several generations -- this is particularly true of the X chromosome. That's because depending on the combination of sexes involved, the X chromosome can sometimes have only half the number of opportunites for recombination as the autosomes do.

Again, I'd say that my siblings and I are at least consistent between our shared DNA and how Ancestry Composition identifies our "ancestral" segments. There are occasional glitches that defy explanation: for example, the Native American segment on chromosome 15 that I mentioned has a different attribution for three of us than it has for the fourth. AC says it's paternal for three of us, and maternal for one, even though it's clearly the same segment. (And our father doesn't even have any Native American segments.)

Even more telling than that is the fact that the segment is supposedly paternal for me, but I also passed it on to my daughter. She didn't inherit any DNA from my father on her entire paternal copy of chromosome 15. The only thing I can figure is that using just one out of six children for all child-phasing allows errors to appear, where the parent ends up not matching the other offspring correctly.

EDIT:

Though it would be more work for 23andMe, when they have six offspring available as they do with my family, they could probably get excellent results with child-phasing. I would guess even better results, sometimes, than when they're able to phase an individual's data using both parents.