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#1




Genetics : Probability Question
A woman who has two brothers with hemophilia A and two normal sons is again pregnant. She requests counseling for the risk of
her fetus to have hemophilia. What is the risk that her next child will have hemophilia? a. 1 b. 1/2 c. 1/4 d. 1/8 e. 1/16 Someone kindly explain this with some good explanation. I ve been getting a lot of such questions wrong on my tests. Whats the concept behind such questions. 
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npktun (05192013) 
#2




50 % 1/2
first we determine the genotype of the mother .... if both her two brothers are hemophiliac then she would definitely would be the carrier .... then every son born to her has a half chance of having hemophilia ....regardless of the no of borns already .... As hemophilia is a Xlinked recessive .... a male fetus needs one defective gene and girl needs two defective genes to have the disease and if a girl is born ....she would have zero percent assuming her husband is not hemophiliac .....As it needs two defective X genes for a girl to be hemophiliac ....and mother is a carrier ....she would pass only one ..... Hope this helps ....and you should also know that the Male fetus receives his X chromosome from the mother only ...and she determines whether her son would be hemophiliac or not ...and as she has two X ....one defective ...so she has a 50 % chance of passing both genes ....
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NIRFA00 (02202014) 
#3




ok, so i'll paste here the answer i read, idk how credible it is, but u tell me if it makes ANY sense to u. Its messing with my concepts a lot.
Here it is : Ans is d. Probability that she is carrier Â½ Probability that she is going to transfer gene is Â½ Probability that the baby boy will get the ds is Â½ So net probability is 1/2 x1/2 x1/2=1/8 NOW, how in the world do they kno that its a male child and not female, cuz they dont mention anything about it in the question. And why is probability of her being a carrier is 1/2, i mean she HAS to be carrier right ? And also whats up with the probability of transfer of gene, whats the concept behind it :/ 


#4




1/4,,,,she is carier and her husban is normal asume,,,

#5




yes it is 1/8..i for got to add her probality of being carrier

#6




Quote:
Is this a UW question ??? Yeah they didn't stated about the sex thing .... .... seems complicated though .... and i am not sure that the last prob needs to be calculated of having the disease ....if they are assuming that its a baby boy ...so once they have calculated the probab of transferring the gene ....this should mean the prob of acquiring the disease
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#7




Quote:
first of all she has not to be a carrier, her mom has to be a carrier, so she has 50% chance of being a carrier. also her fetus has to be boy to get hemophilia by a single x. therefore chance of having a boy is 50%. again chance of transferring the hemophilia x gene to that imaginary boy is 50%. 1/2 x1/2 x1/2=1/8 
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#8




How do we assume that husband is normal and not carrier. And why is probability of her being a carrier 1/2 and not just 1. Cuz, she looks like a carrier for sure, whats up with this half probability. Can u explain why the probability of her being a carrier is 1/2

#9




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Thanks man 
#10




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good luck 
#11




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But wait, m sorry, i really suck at genetics, can u plz explain one last time about why the probability of this mom to be carrier is 50% and not 25 % :/ 
#12




i completely messed it up
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#13




I dont kno any of such questions, i ALWAYS mess up and its bugging me so much now. Whatever i think, they come up with something else. I need to get my concepts down for these kinda questions, and reading book isnt helping tbh. Someone needs to act God here and explain this to me in simple noob terms

#14




Quote:
The mom's brothers both have hemophilia, which means her mom has to be a carrier. The chances she passed the defect gene on to her daughter (= the mom in our case here) are 50% (which would make her a carrier, she would not have the disease). Knowing the fact that her brother's both have the disease, we still don't know if she has it. Of course, her dad could also have the disease, but I am assuming we would know about that, so for this question we will just forget about the dad. 
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nandish_m (05192013), Thrax_usmle (05192013) 
#15




Quote:
so the pregnant lady which is XX got the the first X from her dad and the other X from her carrier mom therefore this second x might be normal or hemophilic. i hope it would help BTW remember it doesn't matter how many hemophilic brothers she has, 1 or 10 doesn't matter at all. 
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nandish_m (05192013), Thrax_usmle (05192013) 
#16




So lets create possibilities ....
Mother having the gene  50 % chance as we know the mother is not hemophiliac so we determine that her father was not a hemophiliac ....because she needs defective gene from both the parents to express the symptoms only her mother was ....so if she is a carrier she has 50 % chance of transferring it Mother having a boy 50 % chance a boy and a girl Mother having transferring the gene  50 % chance SO we assume the question asks us about a hemophiliac fetus ...so we assume they are talking about a boy and not about a girl as a girl will not be affected by a carrier mother .... so this would be a general method to solve an Xlinked disease ???
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#17




Ok cool. I was making that genetics table and then calculating percentage probability of her being a carrier and it was messed up. But anyways, so its 50% probability in the way that she can either get the normal x gene from her mom or the defective x gene from her mom, so it has to be 50% probability.

#18




Quote:

#19




Quote:

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Thrax_usmle (05192013) 
#20




and if they somehow stated that the mother was a carrier
then the probab would be 1/4 right ????
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#22




Yea, according to the concepts above, i this yes. We'd kno for sure that she is a carrier , and we dont need to kno probability of her being a carrier then.

#23




And to seal this off ....
If they stated what would be the prob of A boy then probab should be 1/2
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nandish_m (05192013) 
#24




Quote:

#25




Gosh, u nailed it. Ur the man. (if ur the man, cuz apparently not everyone is man, i just figured that out ) But yea, 1/2 should be it then

#26




yeah ....but we are assuming her husband is clean ..... otherwise they would have stated i think ....
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#27




Quote:
I do however agree that no matter the combination of genotype of the grandparents, the fetus mother has 50 % chance of being a carrier. U can try the various combination. So yes the the mother of the fetus is a carrier for sure but the grandmother can be diseased if the grandfather is normal.
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Fereshteh (05192013) 
#28




Lol .....this time you are right .... now nobody is messing me up with this prob thing
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#29




If anyone has other questions too about other genetics diseases, like autosomal dominant, autosomal recessive and so on, which deal with this probability concept, kindly post them here, so we can discuss it or someone can explain them. It'll be really helpful. Thanks

#30




Quote:
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In USMLE, “Any fool can know. The point is to understand.” 
#31




Q. 2 :
A man whose brother has cystic fibrosis wants to know his risk of having an affected child. The prevalence of cystic fibrosis is 1 in 1600 individuals. The risk in this case is
a. 1/8 b. 1/16 c. 1/60 d. 1/120 e. 1/256 
#32




Quote:

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nsesereso (05202013) 
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