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Old 03-29-2015
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Default Genetics question

A 30 year old man is phenotypically normal but two of his siblings siblings died from infantile Tay sach's disease, an autosomal recessive condition that is lethal by age of five. What is the risk that this man is a heterozygous carrier of the disease causing mutation?

A. 1/4
B. 1/2
C. 2/3
D. 3/4
E. Not elevated above that of general population

Now my question is how do you know the genotype of the parents in this question whether they are homozygous or heterozygous carriers ????
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Old 03-29-2015
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I also have a problem with this question too, how can you comment about the man's parent?

the following can lead the man to be a carieer(Aa) :

AA(man's dad) x Aa(man's mom)
Aa(man's dad) x Aa(man's mom)
Aa(man's dad) x AA(man's mom)

all of the following choices can lead the man to become a carrier(Aa) somehow or the other, but the explanation provided in the kaplan book simply says "his parents both must be carriers ("his" being the man)"
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Old 03-29-2015
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Yep its the same question i really cant make out there can two possiblities regarding either one of the parent.


The punnet squares (two possibilities)

One parent homozygous affected:
A. a.
a. Aa aa

a. Aa aa

Both parents heterozygous carriers:

A. a.
A. AA Aa

a. Aa aa

Both cant be homozygous affected otherwise there's no question of him( the son) being a carrier and Ofcourse both parents cant be homozygous normal =) otherwise there wont be any disease lol

Last edited by Hassan8; 03-29-2015 at 08:51 AM.
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Old 03-29-2015
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Ok i got it !! Cleverfox.. The hint about his parents genotype is in the question itself. Come to think of it its actually a small thing. In the question they say that the disease is lethal in infancy plus we know that its an autosomal recessive so the only way it can manifest is the person is homozygous for it. Either of the parents cant be homozygous OTHERWISE THEY WOULDVE DIED IN INFANCY ! And they wouldnt become parents in the first place !!
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Quote:
Originally Posted by Hassan8 View Post
Ok i got it !! Cleverfox.. The hint about his parents genotype is in the question itself. Come to think of it its actually a small thing. In the question they say that the disease is lethal in infancy plus we know that its an autosomal recessive so the only way it can manifest is the person is homozygous for it. Either of the parents cant be homozygous OTHERWISE THEY WOULDVE DIED IN INFANCY ! And they wouldnt become parents in the first place !!
Yeah got your point but the question aks something else all together...

What is the risk that this man is a heterozygous carrier of the disease causing mutation?

From what you said its clear that the man and his wife both must be heterozygous carriers for their child to die

But the question asks for the man's risk for being a heterozygous carrier!!!!

Basically I cant understand what the question wants us to do...

Can you explain how come the answer given is 2/3
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Yeah that can be calculated by first of all making a punnet square which would be
(Assume 'A' is normal allele and 'a' is diseased allele)

.A. .a.

A. AA Aa

a. Aa aa

So, this person in question is phenotypically normal hence he cannot be 'aa' (which is the homozygous affected and would present as diseased)

He can be either homozygous normal AA(1 possibility in the punnet square)
or he can heterozygous carrier Aa
(2 possibilities in the punnet square). So, Total possibilities are 3... Since the question asks what is the risk of hime being a heterozygous carrier, that would be 2 out of the total 3 possibilities (2/3)
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