How much are siblings related?

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I once read somewhere that, on average, siblings are related by 50% - i.e. if you looked at all of sister A's genes and all of brother B's genes, half would be the same.

Supposedly the amount of relation between you and a family member is [1 / (2^n)] where n is the number of "steps" you'd have to take to get to get to that family member. So, I'm related to my sister (n=1) by 1/2, my grandmother (n=2) by 1/4, my aunt (n=2) by 1/4, and my cousin (n=3) by 1/8.

My biology grad student friend says I'm wrong, but she couldn't really adequately explain why. I'm pretty sure I read this in a Richard Dawkins book. Settle my bet please.

― Cheek0 (Cheek0), Monday, 8 January 2007 19:09 (nineteen years ago)

http://taumoda.com/web/PD/library/kin.html

― Britain's Obtusest Shepherd (Alan), Monday, 8 January 2007 19:11 (nineteen years ago)

me use google

― Britain's Obtusest Shepherd (Alan), Monday, 8 January 2007 19:11 (nineteen years ago)

Oh God!! My mother is my sister!!!

― JTS (JTS), Monday, 8 January 2007 20:42 (nineteen years ago)

omg @ ken laying down the pwnj on jade's mom in the diary room.

― teh_kit (g-kit), Monday, 8 January 2007 21:05 (nineteen years ago)

lol wrong thread sry

― teh_kit (g-kit), Monday, 8 January 2007 21:05 (nineteen years ago)

Aw I thought it fit in okay.

― Abbott (Abbott), Tuesday, 9 January 2007 00:30 (nineteen years ago)

why do i feel like this thread question is the leadup to another, bigger, much more disturbing question

― m@p (plosive), Tuesday, 9 January 2007 00:36 (nineteen years ago)

Ha ha - that was my first thought too.

― Alba (Alba), Tuesday, 9 January 2007 00:42 (nineteen years ago)

THEY 100 PRECENT RELATED DUMASS, THATS WHY THEY RELATIVES

― pumkin (pumkin), Tuesday, 9 January 2007 00:45 (nineteen years ago)

Siblings are on average 50% genetically related, but they aren't necessarily 50%. Whereas your parents are necessarily 50% related to you (in terms of chromosomes - obv. if you are male then more of your genes come from yr momz because the Y chromosome yr dad gave you has fewer genes than the X chromosome yr mom gave you. But if you have Klinefelter or Turner syndrome or some shit and you have extra chromosomes the split won't be 50/50, but that's just splitting hairs)

― Curt1s St3ph3ns, Tuesday, 9 January 2007 00:50 (nineteen years ago)

Curt1s articulated the discrepancy pretty well. I guess what my friend was saying is that "since there are so many genes, it's impossible to just say flat out 50%" but I think the important phrase is "on average."

― Cheek0 (Cheek0), Tuesday, 9 January 2007 01:57 (nineteen years ago)

You know, I always assumed that large parts of the genetic code would be more or less the same for all people -- that it not only accounts for our differences, but also for things that make us similar. And while it wouldn't be universal, perhaps, nevertheless most people would share most of a certain amount of the gene code.

But I don't know if that's actually true or not.

― Casuistry (Chris P), Tuesday, 9 January 2007 02:12 (nineteen years ago)

Well, yeah, that's true. Good point.

― Curt1s St3ph3ns, Tuesday, 9 January 2007 03:40 (nineteen years ago)

It is true! Like the old thing of us sharing 99% of our genes with a chimpanzee. I can't remember how this tallies with the sibling relation percentages. I used to know!

― Alba (Alba), Tuesday, 9 January 2007 10:01 (nineteen years ago)

why do i feel like this thread question is the leadup to another, bigger, much more disturbing question
-- ... (m@p), January 9th, 2007.

yup!

― the original hauntology blogging crew (Enrique), Tuesday, 9 January 2007 10:02 (nineteen years ago)

Oh, I guess it's quite simple really. As Alan's link says, the coefficient of relatedness is "the percentage of genes that those two individuals share by common descent", not how many are actually the same, which will be much more. So you do have most of the same genes as every other human, but when it comes to considering the coefficient of relatedness, if they don't come from the same source, then they're not "shared". Though I guess they all come from the same source ultimately. hmmm...

― Alba (Alba), Tuesday, 9 January 2007 10:05 (nineteen years ago)

the missing link

― mark s (mark s), Tuesday, 9 January 2007 10:38 (nineteen years ago)

!!!

― mark s (mark s), Tuesday, 9 January 2007 10:39 (nineteen years ago)

The initial question that brought us to this had to do with identical twins. Identical twins share 100% of the same genes. If identical sisters Helena and Helenb each have one child, respectively named A and B, A and B will be cousins - but how much will they be related? According to wikipedia:

Genetically speaking, the children of identical twins are half-siblings rather than cousins.

― Cheek0 (Cheek0), Tuesday, 9 January 2007 15:49 (nineteen years ago)

Wikipedia is making a huge presumption there.

― The Android Cat (Dan Perry), Tuesday, 9 January 2007 15:51 (nineteen years ago)

they'd be totally siblings if they had the same father! How about that one!

and if they were conceived on the same day by the same father.. actually hang on.. OK, by identical twin fathers, they'd be the same PERSON!!

― mark grout (mark grout), Tuesday, 9 January 2007 15:59 (nineteen years ago)

WHAT IF THEY WERE ALL CLONES!!!!

― Do Not Feed The Crush (kate), Tuesday, 9 January 2007 16:00 (nineteen years ago)

WHAT IF THEY WERE ALL MICK HUCKNELL????

― mark grout (mark grout), Tuesday, 9 January 2007 16:03 (nineteen years ago)

this thread is making me horny

― ken c (ken c), Tuesday, 9 January 2007 16:14 (nineteen years ago)

Shocker.

― The Android Cat (Dan Perry), Tuesday, 9 January 2007 16:16 (nineteen years ago)

hehe i can't stop finding this diagram hilarious
http://taumoda.com/web/PD/library/rexample.jpg

― ken c (ken c), Tuesday, 9 January 2007 16:16 (nineteen years ago)

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