alex-)
Forum Newbie
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Ok, I am sorry may be it was a little vague...
The probab = 1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1/32
why is there only five 1/2s and not six?
consider this as 2 branches of the family tree from common GGP
each 1/2 represents 50% chance of passing gene down
that's it (1/2)^5, not (1/2)^6 because GGP chances of passing it to either GP is 1/2 not 1/2 * 1/2!!!
GGP
1/2 GP 1/2 P 1/2 Child
GP
1/2
P
1/2
Child (who are 2nd cousin to the other child)
Hope this clears
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| docofthebigapple
Forum Senior
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The more anyone explains, the messier it gets.
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| Malaysian
Forum Guru
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actually not so docoftthebigapple......as a matter of fact what alex-) had said is the same as what I or vrach had mentioned.I realised this after sending my last post and reading alex's last post.
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| iwonder
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Malaysian,
it's unrelated to the original question, but could you please elaborate on "1/2 of sibling genes are shared" for me? a child gets one half of its mother genes and one half of its father genes. there is no guarantee its brother/sister will get either of the same halves from the parents. if anything, i'd say that roughly 1/4 of the siblings will have the same genes
or did i take your statement out of the context?
thanks!
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| iwonder
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alex-) and uscan,
what if we look at the question this way:
the boy's 2nd cousin is affected :arrow: her parent of the boy'side has 1/2 chance of being affected :arrow: her grandparent on the boy's side has 1/4 chance of being affected. that grandparent and the boy's grandparent are siblings; we know that they have the same parents (they would be the boy's great grand parents), so that would give them both 1/4 chance of being affected :arrow: the boy's parent has 1/8 chance of being affected :arrow: the boy has 1/16 chance of being affected
that sends it into another universe now. where is the mistake in my reasoning?
thank you!
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| medstudent30
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uscan, I think your solution is amazing.
iwonder, your approach is very interesting, but I think I can see why that wouldn't be the correct answer, not to take anything away from your reasoning. If you have two siblings, they should have an equal probability of being affected with any particular condition or having a gene. However, we are given more information because we know that the descendent of one of the siblings is affected and we do not have any information about the descendents of the other person. Because we have this extra piece of information, it would make one sibling have a higher probability of being affected. Is my reasoning right?
Malaysian, would you be able to explain how half of sibling genes are shared? I'm curious about this point as well. thanks.
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| iwonder
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medstudent30,
i would agree with you, but we don't know which side of the family the affected second cousin got the disease from. it could be from the great grandparent on the boy's side, but it could be from 7 other great grandparents
besides, even though the grandchildren of the famous Mendel's pea plants did split their height trait 1/3 (1 short plant to every 3 tall plants), all of them had an equal probability of getting a particular trait, as siblings. that's the whole point of Mendel's experiment (his law of independent assortment)
as far as uscan's explanation, he/she states in one place:
"If the second cousin is affected, the probability that her parent from the boy's side is affected: 1/2
the proba that her grand parents from the boy's side are affected is 1/2)(1/2)=(1/4)
That means, the proba for the great grand parents to be affected=1/4(because one of their children is affected!)"
and in another place:
"when someone has a disease that is known to be AD, each parent has a proba(1/2) to have the D and each grand parent of the diseased parent has (1/2) proba to be affected "
so where is it - from the Parent to the Grandparent, or from the GP to the GGP that the probability stays the same instead of splitting in 2? and, since we are allowing that, why does it only happen in one link? what stops us from keeping the 1/2 probability up until the GGP?
now, just for an exercise of logic: imagine that the affected second cousin is homozygous; it's not very probable, but possible. that means that she got her disease from both parents. that would make the disease probability for both of them 1, not 1/2. it gets more and more confusing if we operate with assumptions instead of what we can know for certain
sounds like there is not enough information to go by to justify the 1/32 answer. i guess that's the curse of working backwards: building a solution from an answer
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| Malaysian
Forum Guru
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Posts: 778
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Hi iwonder,
As to why siblings share half their genes....assume the father has genes A &B.Mother has genes B&D.....if these genes were to randomly distribute into children the possible outcomes are:
AC,AD,BC and BD......notice that any of the mother's or father's genes A,B,C,D when distributed randomly is present in only half of the possible genetic makeup of the kids.
so lets take AC as an example.....if AC had a sibling with AC make up that will be 100% same genetic makeup.If AC had AD as a sibling there will be 50% same genes.Ac had BC as sibling its 50% again and if AC had BD 0% same gentic makeup....if you take the average 100%+50%+50%+0%/4 it comes to 50%
in regards to your second query,your answer would be correct if the GGP's genes was to be transferred down to the boy....however the question is asking for 2 family lineage.....so your calculation must include 2 family lineage.The spouse who gets married into family lineage causes only 1/2 genes to be tranferred to the next generation.To elaborate further...
siblings share only half their genes because they are of the same lineage...same parents.
for cousins its 1/2(because cousin's parent(s) are brother/sister)*1/2(for lineage 1)*1/2(lineage 2)=1/8
for second cousins its 1/8(because parents are cousins)*1/2(lineage 1)*1/2(lineage 2)=1/32
so if it was next generation(is it called 3rd cousins??)it would be 1/32(because parents are second cousins)*1/2(lineage 1)*1/2(lineage 2)=1/128
How about an uncle with his niece??
well niece's mum/dad is uncle's sibling so they share 1/2 genes and 1/2(because lineage 1) so 1/2*1/2=1/4
I'm sorry to cause any confusion in this already confusing topic!!....my explanation would work best if you have an empty piece of paper and you draw a rough family tree to really see whats happening!
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| iwonder
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Malaysian,
you've made the most sense out of everybody (including myself) 
after i got what you meant by "siblings share half their genes" (which in our case is probably better to phrase as "there is a 1/2 probability that siblings share one particular gene"), it all became clear. in my first post i DID consider 2 family lineage, i just assumed that the siblings' probabilities were the same, which was wrong. and that's where the extra 1/2 was hidden
thank you for taking the time to expain this. truly!
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| royalgeorge
Forum Elite
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This sounded the most logical.
Roy
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| endrit
Forum Newbie
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girl sick p=1
parent from boy side p=1/2
grandparent from boy side p=1/4
greatgrandparent p=1/8 but boy and girl have both the same grandparents so betwen them the chance of one of them havin the gene is 1/8+1/8=1/4
grand parent of boy p to inherit the gene =1/2
boy's parent p=1/4, boy p=1/8 to have inherited the gene from greatgrandparent
so 1/8*1/4=1/32
hope is clear enough
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