








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




Belindalimm Daily Questions: Genetics #1
A 14yearold 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. 
The above post was thanked by:  
Casandra (08202012) 
#2




my answer :)
I would assume that her husband is not affected.
I would say this disease is Xlinked 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 
#3




Quote:

#4




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 
#5




they gave 1/8 as the correct answer?

#6




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. 
#7




.
Quote:

The above post was thanked by:  
belindalimm (08202012) 
#8




Quote:

#9




I got it now

#10




Yup. The correct answer is 1/8.

#11




where does the 1/4 come from?

#12




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 Xlinked 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.

#13




Quote:

#14




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

The above post was thanked by:  
hoor (08212012) 
Tags 
Genetics, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Belindalimm Daily Questions: Microbiology #1  belindalimm  USMLE Step 1 Forum  8  09182012 03:42 AM 
Belindalimm Daily Questions: Biochemistry #2  belindalimm  USMLE Step 1 Forum  10  08242012 04:01 AM 
Belindalimm Daily Questions: Biochemistry #1  belindalimm  USMLE Step 1 Forum  2  08232012 09:57 PM 
Belindalimm Daily Questions: Histology #1  belindalimm  USMLE Step 1 Forum  8  08222012 01:19 PM 
Belindalimm Daily Questions: Embryology #1  belindalimm  USMLE Step 1 Forum  14  08192012 09:57 PM 
