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Discussion Starter #1
A 14yr old experienced prolonged severe bleeding after having a tooth extracted. He also has a history of multiple episodes of joint swelling following minor trauma. His parents have no bleeding problems. Evaluation reveals that the patient has an inherited disorder. Now, his older sister who does not have this condition, is pregnant (sex of the child unknown) and asks about the risk that her child will be affected. The best probable estimate is.

A) Near 0
B) 1/2
C) 1/4
D) 1/8
E) 1/16
F) 1/32
 

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Hemophilia (X-linked recessive)

ANSWER: D) 1/8.
Here we're talking about hemophilia (probably A) which is an X-linked recessive condition (Factor VIII:C deficiency) in which patients suffer episodes of bleeding and multiple hemarthroses. If the boy has the disorder, it means that her mother gave him her X mutated chromosome. We know that his sister doesn't have hemophilia, but does she have the X mutated chromosome? In other words, is she a carrier? -We don't know, so prob: 1/2. Did she gave her X mutated chromosome to her offspring? -She has two X chromosome but can only give one, so prob: 1/2. And the last question is: What is the sex of the child? (because we know that an X-linked recessive condition will only manifest itself in male patients). -We are said that the sex of the child is unknown, so prob of being male: 1/2.

Then, the best probable estimate is: 1/2 · 1/2 · 1/2 = 1/8
 

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if mom is the carrier doesnt that mean she will give the mutated chromosome to half of her children ... so i was thinking it would be 1/4 .....plzzz explain me i didnt get the last two 1/2's
 

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If Mom's the carrier, then she will give it to half of her children, as you say, however, only 50% of her children will express the phenotype as only males express the phenotype. so 1/4 * 1/2 = 1/8

I think this is a UW question. I've seen something identical before...

if mom is the carrier doesnt that mean she will give the mutated chromosome to half of her children ... so i was thinking it would be 1/4 .....plzzz explain me i didnt get the last two 1/2's
 

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Discussion Starter #7
its from UW.
The correct answer is 1/8(D). Their explanation was a bit brief.
This clears it up now.
Thanks guys.

If Mom's the carrier, then she will give it to half of her children, as you say, however, only 50% of her children will express the phenotype as only males express the phenotype. so 1/4 * 1/2 = 1/8

I think this is a UW question. I've seen something identical before...
 
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