bobby
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A boy from a country in war (ex. Afghanistan) goes to see his doctor because he is worried about an autosomal dominant disease. The disease is present in one of his female relative, they share the same great grand parents (i.e second cousin). The boy is unable to give any other information about his family history saying that the rest of his family died during the war. Given this information what is the probability of him having the disease?
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| anne
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1/16th :?:
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| bobby
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anne, acording to the ans given for that question,yours is not correct.
unfortunately, there was no explanation accompanying the ans. so i cannot explain why your ans is wrong.
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| merjr
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1/8...oops that was for 1st cousins
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| anne
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what's the answer?
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| bobby
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the right answer is 1/32,
but i don't understand how they got it
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| asmi
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Since 50% chance of him being a carrier( 1/2)
He and his second cousin shares same great grand parents...(1/2) 5 = 1/32
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| merjr
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thats clear now ..Thanks
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| uscan
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I don't get it, what the 5 stand for?
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| asmi
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its to the power of...1/2 x1/2 x12x 1/2 x1/2
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| uscan
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Yeah, but do you mean there is 5 generations, I think there is just 4: the cousins-their parents-the grand parents and the great grand parents!??
:?
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| asmi
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I may be wrong then 
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| uscan
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This question kept on bothering me. I finally got the explanation. I don't know if there is an easy formula but this is how I got it:
To find the risk of this patient we have to find first that the great grand parents are affected: 1/2(probability for the cousin's parents) multiply 1/2(the proba for the cousin's grand-parents)=1/4,
When the g.g.parents are affected, the probability that the patient is affected=1/2(for the grands parents)multiply 1/2 (for his parents)multiply 1/2 (for himself)=1/8
now the probability for him to have the disease knowing just about his cousin: (1/4)(1/8)=1/32.
Hope I am right!
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| asmi
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uscan i didn't understand your method . :oops:
Is there any reference ..?
I was thinking that "second cousin" is a hint"..
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| uscan
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I will try to explain in other ways:
In this case all we know is that the second cousin is affected and that she shares the great grand parents with the boy in question.
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!)
Now, for the boy:
If his great grand parents are affected, the proba for him to be affected
is: (1/2)(1/2)(1/2)=1/8
Because we are not sure that the great grand parents are affected but there is a proba:1/4 that they are :arrow: the proba for the boy is1/8 )(1/4)=1/32.
Hope this helps
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| bobby
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thanx, uscan. :idea:
this prob seems to be a dilemma.
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| anne
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uscan,i may sound stupid for not being able to get it.but as both 2nd cousins share the same gr8 grandparents,i thought both shall have equal probability of having the disease,so shouldnt it be1/8x1/8=1/64.i didnt understand when u said that:"That means, the proba for the great grand parents to be affected=1/4(because one of their children is affected!)".i'll appreciate if anyone can clear it for me.
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| uscan
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Anne, when someone has a disease that is known to be AD :arrow: 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 :arrow: the proba that the GP of the same lineage is affected:1/4 and this represent the proba that the GGP have the D because their son/daughter(which are the GP) are affected. :| Did I clarify anything :?:
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| anne
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yes,thanx lot!!
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| asmi
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:cry: :cry:
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| Malaysian
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I think it has nothing to do with any calculation!Even if the question had mentioned it was an autosomal recessive disease the answer would have remained the same.
The explanation given above regarding the calculation makes no sense!
This is because while 1/2 of sibling genes are shared,1/8 of cousins share the same genes,1/4 of genes are shared between uncle-niece and though I haven't read it but most likely 2nd cousins share 1/32 of their genes making the probability of any genetic disease 1/32 among second cousins!
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| alex-)
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it's possible to do math on it.
There was one calculation which got 1/64 (1/8 * 1/8) with one little mistake, on the top of a tree you have 1 parent with prob of passing D down 1/2 but in calculations you include it 2 times, thats why it's 1/32 not 1/64
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)
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| Malaysian
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I'm sorry alex....I didn't understand what you are trying to get at.....can you systematically show me how you got the answer.Why is it multiplied by 2??
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| vrach
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Malaysian, I totally agree with your approach to this Q. The bottom line to this Q is "what fraction of genes are shared among 2nd cousins?", and one of those genes might just happen to have the mutation!
But, sholdn't it make a difference if the diseae in the Q was an AR disease? Cos, only one allele is inherited from the common ancesters. The other would be from the population gene pool, and this would definetly impact the probability of inheriting an AR disease.
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| Malaysian
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Yes.....I suppose so....but if the question is as vague as above and says nothing about itself....I'll just go with the answer that says'there's no way of calculating it' and if that isn't there then 1/32!
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