Luay
Forum Newbie
Topics: 4
Posts: 5
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a child knows to have Tay-sach's disease, what is the probablity his Grandfather is a carrier?
A. 0%
B. 25%
C. 50%
D. 75%
E. 100%
Please elaborate and explain. (those questions drive me nuts!)
Thank you
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| Ancylostoma
Forum Guru
Topics: 42
Posts: 642
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My reasoning says its 75%. If you have the answer then tell me and if Its right then I will explain my reasoning. Theres no point explaining an incorrect answer.
IF it matters, i'm almost 90% sure
Edited by Ancylostoma on 12/28/06 - 04:14 PM
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| mol genetics
Forum Newbie
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Posts: 1
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a child knows to have Tay-sach's disease, what is the probablity his Grandfather is a carrier?
A. 0%
B. 25%
C. 50%
D. 75%
E. 100%
Answer, B (i think)
Ok Let us presume that gene for beta-hexosaminidase A enzyme is called A.
Tay sachs is a reccessive disease so child must be aa.
Note that tay sachs children dont (normally) live to reproductive age so this means that child's parents are Aa for his father and Aa for his mother. they both must be carriers for the child to be afflicted with Tay Sachs which as we said is a recessive disease.
Now, if the father is Aa then the grandfather must either be AA or Aa and the grandmother must also be either Aa or AA.
Now if great grandfather is also either AA or Aa and great grandmother the same.
so if great grandfather is AA and great grandmother is Aa then progeny is AA, Aa, Aa, Aa. so there is a 50& chance of great grandfather's children of being carrier. This is also the case if great grandfather is Aa and great grandmother is AA.
The chance of being a male child is 50%....
So, chance of grandfather being a carrier is 50/100 * 50/100 = 0.25
Thus, chance grandfather of being a carrier is 25%.
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| Ancylostoma
Forum Guru
Topics: 42
Posts: 642
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your doing something wrong mol, I dont think you calculated for the fact that he has two grandfathers. The questions is more complex. the probability that one granfather is carrier is 50%, because either him or his wife have to be a carrier to pass the gene on. Then there are two grandfathers. so basically its asking you what is the probability that either grandfather is a carrier. So you add 50% +50% (the prob that each grandfather is a carrier) and you then subtract the probability that both grandmas are carriers ( .5*.5) .25%. So the probability that his granfather is a carrier is 100%-25%= 75%
mol you didnt find 25% funny with two grandfathers present ? it not asking the chance of both grandfathers being carrier, either !!
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| Ancylostoma
Forum Guru
Topics: 42
Posts: 642
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I just thought of a way to clarify the problem. If you flip a coin you have two possible outcomes, .5 chance of head , and .5 chance of tails . What is the probability of of getting either heads or tails ? .5+.5 = 100%
In the problem above, we are flipping two coins. What is the probability of flipping a head. .5+.5 (probability of flipping heads) - the probability of getting tails on both .5*.5=.25 . Thus your answer is 1-.25= .75 or 75%
If it asks you the probability of flipping both heads then the answer is .5*.5=.25
You should change you screen name and take the genetics part out. Its misleading 
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| brutus25
Forum Junior
Topics: 13
Posts: 62
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Tay sachs is a reccessive disease so child must be aa - Correct
Father - Aa Mother - Aa - Correct
The chance of being a male child is 50%.... What is this? We are talking about a AR disease not XR.
In my oppinion we should just focus on the possible outcomes between grandfather and grandmather not greatgrand...
Now , for the father to be Aa --> possible combinations :
Grandfather Grandmother
AA Aa
Aa AA
Aa Aa
Now you can see that for the father to be Aa , grandfather can have only 2 possible options ( what I`ve just described up there are actual combinations , you can`t say that the grandfather actually has 33%chance of beeing AA , 33% Aa and 33%Aa this would just be nonsense) grandfather , as well as grandmother , could be either AA or Aa.If you still want granfather to be a carrier there is still 50% chance of beeing one.
Answer C
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| Ancylostoma
Forum Guru
Topics: 42
Posts: 642
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Your clearly did not consider the fact that he has two grandfathers and the question is asking you the probability that either one is a carrier.EITHER EITHER EITHER EITHER!!!!!!! I tried to explain the answer, but without a good grasp on probability theory , you cannot answer this one correctly. The answer is D
Edited by Ancylostoma on 12/31/06 - 04:21 AM
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| Luay
Forum Newbie
Topics: 4
Posts: 5
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I agree with brutus25.
C. 50%
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