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Old 08-20-2012
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Arrow Belindalimm Daily Questions: Genetics #1

A 14-year-old boy experiences severe prolonged bleeding following a tooth extraction. He also has a history of multiple episodes of painful joint swelling following minor trauma. His parents have no bleeding problems. Evaluation reveals that the patient has an inherited disorder and that one of his parents is a genetic carrier. Now his older sister, who does not have this condition, is pregnant(sex of the chid unknown) and asks about the risk that her child will be affected. The best probable estimate that her child will have the disease is:

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

I personally think of the question lacking of the information of the sister's husband, say if he has the bleeding problem, since if the condition of her husband is included, it will yield a different outcome.

What do you guys think? Welcome all of your opinions and questions.
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Default my answer :)

I would assume that her husband is not affected.

I would say this disease is X-linked recessive (hence only one of the parents - mother - is a carrier but none of the parents is affected and still the fruit of this relationship - boy - is affected)

So the sister has 1/2 chances of being a carrier of this recessive gene. Again, I assume that husband is healthy. So with this combination only their baby boy can be affected:
That would give us:
1/2 x 1/4 = 1/8

So I'll go with D. 1/8
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Quote:
Originally Posted by Casandra View Post
I would assume that her husband is not affected.

I would say this disease is X-linked recessive (hence only one of the parents - mother - is a carrier but none of the parents is affected and still the fruit of this relationship - boy - is affected)

So the sister has 1/2 chances of being a carrier of this recessive gene. Again, I assume that husband is healthy. So with this combination only their baby boy can be affected:
That would give us:
1/2 x 1/4 = 1/8

So I'll go with D. 1/8
Yup, the odd is 1/8. When I solved the question, I also assumed his husband is normal. But when I thought further, it could be possible his husband is with this condition, in that case the baby would be at 1/4 chance of being inherited the disease(boy and girl in total).
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Old 08-20-2012
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i will go for Eee...1/16

chance of her sister having this trait is 1/4
chance her current pregnancy will be boy is 1/2
chance of the boy being a carrier is 50% ie 1/2


*its obvious her husband is dx free
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they gave 1/8 as the correct answer?
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Quote:
Originally Posted by imgchuchu View Post

chance of her sister having this trait is 1/4
it's 1/2
we already know she a female so she can be 1 out of 2 possible female outcomes. 1 of them is a carrier. I used to make this mistake

If they didn't say whether she's a sister or a brother (just a sibling) we would calculate the risk by dividing by 4, but since we know her gender we divide it by 2.
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Quote:
Originally Posted by belindalimm View Post
Yup, the odd is 1/8. When I solved the question, I also assumed his husband is normal. But when I thought further, it could be possible his husband is with this condition, in that case the baby would be at 1/4 chance of being inherited the disease(boy and girl in total).
I know, I totally get your point but in the questions I have done so far, the assumption always was that if they don't mention anything about a spouse, they're disease-free
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Quote:
Originally Posted by Casandra View Post
I would assume that her husband is not affected.

I would say this disease is X-linked recessive (hence only one of the parents - mother - is a carrier but none of the parents is affected and still the fruit of this relationship - boy - is affected)

So the sister has 1/2 chances of being a carrier of this recessive gene. Again, I assume that husband is healthy. So with this combination only their baby boy can be affected:
That would give us:
1/2 x 1/4 = 1/8

So I'll go with D. 1/8
I agree it's D but can you tell me please what does the 1/2 and 1/4 you mentioned represent? Do you say that the 1/2 represents the risk of the mother being half carrier or the fact that her baby is of unknown gender?
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I got it now
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Quote:
Originally Posted by imgchuchu View Post
they gave 1/8 as the correct answer?
Yup. The correct answer is 1/8.
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where does the 1/4 come from?
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Originally Posted by pathophysio1 View Post
where does the 1/4 come from?
It is 1/4 because the sister is carrier and assuming his husband is healthy, in this case the children from them will only have a 1/4 chance to gain the disease. It is X-linked recessive disease. The question stem also mentions no differentiated sex can be told at this point. So the total odd for offspring including boys and girls to get the disease is 25%. Hope my explanation erase your concerns.
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Quote:
Originally Posted by belindalimm View Post
It is 1/4 because the sister is carrier and assuming his husband is healthy, in this case the children from them will only have a 1/4 chance to gain the disease. It is X-linked recessive disease. The question stem also mentions no differentiated sex can be told at this point. So the total odd for offspring including boys and girls to get the disease is 25%. Hope my explanation erase your concerns.
but we dont know for sure if sister is carrier right?she could be normal too?so chances of her being carrier is 1/2 and in that condition chances of the baby being affected is 1/4 assuming father is normal.so 1/2*1/4=1/8.??
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but we dont know for sure if sister is carrier right?she could be normal too?so chances of her being carrier is 1/2 and in that condition chances of the baby being affected is 1/4 assuming father is normal.so 1/2*1/4=1/8.??
Yup, you're right. The sister has 1/2 chances to be a carrier, in this way, assuming her husband is normal, the odd of her baby to be with the disease is 1/2*1/4=1/8
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