DNA Lesson: Identity By State/Population, Sticky Segments, and Pile Up Regions

[Cheri Mello]

I had a message in my inbox this morning from a cousin who was playing around on GedMatch with their tools. He took the advise of another cousin and changed the parameters down to 1 centiMorgan (cM), as he wanted to see if his parents were cousins.

Do NOT change the parameters down to 1 cM. You're getting into the danger zone!!

My cousins were thinking that changing the default down to 1 cMs would show the "sticky segment phenomenon." There's no such thing as a "sticky segment phenomenon." There are sticky segments however, but I think my cousins were confusing it with other DNA terms.

Identity By State (IBS) or Identity By Population (IBP) means that you come from common Iberian stock. These are the real tiny (1 cM, 2 cMs, etc) blocks. If you are comparing yourself on any type of chromosome browser (whether that's on FTDNA, GedMatch, etc), with only a few other people, you won't be able to see it. It's that (American?) saying "You can't see the forest because of all the trees." You'd have to back up and get a bird's eye view. I use Wes Erickson's Autosomal DNA Utility (I'll just refer to it as the Utility) to back up to the bird's eye view to see the forest.

This is my dad's Chromosome 11. It is ALL of his matches who happen to match him on Chr. 11. That's 792 people, but the Utility can only show 418 people, so that is what you are seeing. Where I marked the X is a pile up region. It's NOT a sticky segment. It's probably Identity By State (IBS) or Identity By Population (IBP).

If you are cranking the parameters down to 1 cM with you and a handful of other people, you can't tell it's IBS/IBP or a pile up because you don't have enough people in order to tell. I think with 418 people, you can tell it's a pile up or IBS/IBP.

As for the sticky segment....

Cousin Charles has an 8.41 cM segment (marked in the picture) with the exact same start and stop positions. I probably have it too, but it's hard to tell since you see so much of Chr. 21 in common between me and my dad. Research (paper trail) led my dad, Charles, and Steve back to Moses Dameron b. 1695. Yet cousin Nancy (also a Moses Dameron descendant) doesn't have it. That's because a sticky segment is an all-or-nothing deal. You inherit in full or not at all. You don't get half (in general, we get half of mom's DNA and half of dad's DNA). You get it all or nothing. Nancy got nothing. Did Nancy's mom have it? Maybe, but she died before DNA testing came about.

Hope this clarified things. I tell people NOT to turn the parameters down to 1 cM. You're in the danger zone and it's misleading. Even 5 cMs can be misleading at times. Just stay with the defaults unless you have a paper trail with a few people matching the same ancestor.

Cheri Mello, Family Tree DNA Admin (volunteer)

[Richard Francis Pimentel]

Good points Cheri!  Thanks for the analysis.

 

Rick

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[Cheri Mello]

There's so much vocabulary to learn with DNA. And some of it doesn't make sense until it happens to you. Hopefully, a new term or two can be learned so next time the explanations will make a little bit more sense.

Cheri Mello
Listowner, Azores-Gen
Researching: São Miguel island: Vila Franca, Ponta Garca, Ribeira Quente, Ribeira das Tainhas, Achada