








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




Complicated CF Genetics Pedigree
How do I do this?
Joan and Claude come to you seeking genetic counseling. Claude was married before, and he and his first wife had a child with cystic fibrosis, an autosomal recessive condition. A brother of Joan’s died of cystic fibrosis and Joan has never been tested for the gene. If they marry, what is the probability that Joan and Claude will have a son that WILL NOT be a carrier for or have cystic fibrosis? A. 1/2 B. 1/4 C. 1/12 D. 1/6 E. 1/8 
The above post was thanked by:  
eveli55 (02132013) 
#2




b. 1/8????
Oh God I am bad at this.:sorry: Last edited by Doc4Step1; 02122013 at 10:35 AM. 
#3




I guess thats the right one
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#4




How did you get 1/8

#5




Quote:
He is either contributing or not the diseased gene to generation = 50%diseased/50%healthy gene goes to offspring So we take 50%=1/2 for healthy gene Now for female partner we dont know if she is a carrier or not so we take the same chance 50/50% so 1/2 chance that she is healthy and in case if she is carrier50% chance that she is transferring healthy gene to child which totaly equals 1/2x1/2x1/2=1/8 but not sure if thats the way
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#6




Answer says it is 1/4. Can some one help.

#7




Can u post the explanation of that question
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#8




" a son that WILL NOT be a carrier for or have cystic fibrosis"
I was waiting on somebody to say 1/4 cause that what i thought the answer was. My reason: cf is autosomal recessive. needing 2 parents which are carriers to produce a child with cf at 25% of the time. based on the q stem, we know that both parents are carriers. we know that because Claude already had a CF child in his past marriage. Joan brother died from CF so we can assume she's a carrier. The questions is asking about what are the chances of producing a Normal child? CF child = 25% CF carrier = 50% Normal child= 25% thats why i picked 1/4. 
#9




Answers 1/4.
I think it's because u guys forgot to take into account that both of Joan's parents must be carriers in order for her brother to be affected. Which mean's that there's 1/2 chance of joan being a carrier and 1/4 chance of her being homozygous for the normal alleles. If you calculate the probability of joan having a homozygous normal child with her carrier husband you'll get two probabilities. One if joan was a carrier (1/2) and the other using joans probability of being homozygous normal (1/4). If you add both probabilities, you'll get 1/8 + 1/8 = 1/4
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#10




Btw where'd u get this question from??
Cuz I think CF has complete penetrance and the answer should be 1/3. Does the book ur using have an erratum?
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#11




This question is definitely pain in the ass
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#12




dont think about it that way. I use to always freak out when i see these kind of answer choices.
But now i just relax and think it through with some deductive reasoning. You have all the information you need on the stem. 
#14




Quote:
Quote:
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#15




Quote:
Thanks in advance
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#16




Quote:
Since Claude had a kid with CF, we know for sure that he's a carrier. Since Joan had a brother with CF, we know her parents must've both been carriers. This means that Joan can either be homozygous normal (CC), or a carrier (Cc), but she can't be homozygous affected (cc) since she obviously doesn't have CF. Knowing that Joan's parents were both affected, we can calculate the probability of Joan being Normal/(CC) or a Carrier/(Cc). If we do a punnet square with Joan's parents (two carriers) we get the values (CC) = 1/3 and (Cc) = 2/3 for Joan (the punnet square would only have 3 possibilities since we already know Joan's not affected and therefore the probability of cc is nonexistant) Now that we have both of Joan's genotypic probabilities, we can calculate the probability of Joan's child with Claude. We'd do this in 2 steps. 1) If Joan were a carrier (probability of 2/3), the probability her child would be homozygous normal (CC) would be: 2/3 * 1/4 = 1/6 2) If Joan were homozygous normal (probability of 1/3), the probability her child would be normal as well: 1/3*1/2= 1/6 Adding both results gives us the true probability of Joan and Claude having a homozygous normal child: 1/6 + 1/6 = 1/3.
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. Last edited by slowpoke; 02132013 at 10:41 AM. 
#17




Quote:
1) If Joan were a carrier (probability of 2/3), the probability her child would be homozygous normal (CC) would be: 2/3 * 1/4 = 1/6 The chance of her giving diseased/normal allele would be 1/2 Which should equal 2/6=1/3
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#18




Quote:
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
The above post was thanked by:  
luschka (02132013) 
#19




Quote:
1/2x(1/3+1/3)=1/3 Much thanks ur post was very helpful
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#20




claude is a carrier
Joan has 2 probability 1carrier 1/2 2normal allel 1/4 claude(carrier) +Joan(carrier) = 1/4 chance for normal not carrier child Claude(carrier)+ Joan(normal) = 1/2 chance for normal not carrier child it is not separated event so we have to multiply 1/4 *1/2=1/8 This is what I think. correct me 
The above post was thanked by:  
Easy (05082013) 
#21




Quote:
Its not as simple as it appears
__________________
Everything is possible for him who believes (MARK 9:23) 245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad 
#22




what's the correct answer?
so...what's the consensus guys?

Tags 
Genetics, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
pedigree chart genetics question  wertheworld11  USMLE Step 1 Forum  6  11192012 11:06 PM 
What is the diagnosis of this complicated patient?  m82_ghasemi  USMLE Step 1 Forum  9  09272011 12:48 PM 
A complicated case of a smoker, carpenter, and alcoholic man!  aghammoud85  USMLE Step 2 CK Forum  4  05312011 04:00 PM 
Complicated unlicensed abortion  RRMadukha  USMLE Step 2 CK Forum  1  05262010 03:49 PM 
