Please help me decipher this: Half Sibling?

+3 votes
112 views

Largest segment = 111.1 cM

Total Half-Match segments (HIR) = 2099.6 cM (58.545 Pct)
Estimated number of generations to MRCA = 1.4

47 shared segments found for this comparison.

466443 SNPs used for this comparison.

60.679 Pct SNPs are full identical

in The Tree House by Jill Shaw G2G Rookie (220 points)
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2 Answers

+2 votes
At 2099 shared cM, according to the DNA Painter calculator, 86% chance this person is your grandparent, aunt/uncle, half-sibling, niece/nephew, or grandchild. There is a 14% chance this is your full sibling.
by Jessica Key G2G6 Pilot (201k points)
+2 votes

Only full siblings share significant fully identical regions. The question is, is the 60% fully identical SNPs “significant”? It sure sounds like it is. But it’s hard to tell, because by descent from common parents you will only be fully identical on 25% of your genome. The reason they report full matching on over 50% of the genome is that most humans match on each spot, with only a minority of people having a differing letter at those spots. With a random person, you will match at  tons of positions just by chance, and not because of kinship.

No one seems to have reported appropriate values for the “pct fully identical” stat for full and half-siblings. So I don’t know for sure, but 60% sure sounds like a lot, so I’m guessing the relationship is full siblings.

But to be sure, run the comparison and look at the graphical output of the chromosomes. If you have lots of blue areas, but also some green areas, then the green areas are fully identical and you are full siblings. If all you see are blue bars, then you are half-siblings.

Edit: Okay, I compared to confirmed full-siblings at GEDmatch. They show 72.5% SNPs are full identical. So now your 60.679% SNPs are full identical seems like it might be low for full siblings. I would definitely run the 1-1 comparison and look at the graphical chromosome depictions.Here is a bar from the comparison I ran:

This represents one pair of chromosomes being compared. Like both of your copies of chromosome 22 being compared to both of your match's copies.

Underneath, the blue bar represents the part where the two people being compared match on at least one of the two chromosomes. The black bar is where neither chromosomes match.

But the blue bar doesn't show you if both chromosomes of the first person are matching both of the chromosomes of the second. That's a fully identical region . If only one chromosome of the first person is matching one of the chromosomes of the second, that is a half identical region.

But look at the colored bar along the top, with all the red, yellow and green. See the solid green region? Any region of solid green is a region where both testees are fully identical, so for a long stretch like this it is clear evidence that they got matching DNA from their fathers and also from their mothers. Except with rare relationship types, that would mean they are full siblings.

So, if you do the comparison and you see a bunch of lengthy green blocks like that, the testees are full siblings. Otherwise, you shouldn't see any at all, and they are half-siblings.

 

by Barry Smith G2G6 Pilot (219k points)
edited by Barry Smith

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