genetics ques!! - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 1 Forum

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


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 08-26-2012
USMLE Forums Newbie
 
Steps History: Step 1 Only
Posts: 9
Threads: 3
Thanked 1 Time in 1 Post
Reputation: 11
Genetics genetics ques!!

A woman who is a known heterozygous carrier of a PKU mutation marries her half second cousin . What is the probability that this couple will produce a child with PKU??

A.1/16

B.2/9

C.1/64

D.1/256

E.1/128
Reply With Quote Quick reply to this message



  #2  
Old 08-26-2012
USMLE Forums Scout
 
Steps History: Not yet
Posts: 72
Threads: 8
Thanked 11 Times in 11 Posts
Reputation: 21
Default

Wow half second cousin

Could you please tell where u picked up this Q from?

Thanks!
Reply With Quote Quick reply to this message
  #3  
Old 08-26-2012
USMLE Forums Master
 
Steps History: 1+CK+CS
Posts: 1,039
Threads: 189
Thanked 557 Times in 318 Posts
Reputation: 567
Default my answer

I think it's D 1/256 but it's based on many assumptions so really not sure...

my calculations:
1/2 x 1/4 x (1/8 x 1/2 x 1/2) = 1/256

1/2 - probability that the husband is a carrier (we have to include that, right??? I mean I assume that he's not affected since the stem doesn't say anything and if he's healthy then they will not have a child with PKU but not sure...)
1/4 - probability that, provided both parents are carriers, they will have a child with PKU

1/8 - pool of genes their grandaparents shared (half first cousins)
x 1/2 x 1/2 - portion of genes passed on through 2 generations (from grandparents to the present couple)
Reply With Quote Quick reply to this message
  #4  
Old 08-26-2012
USMLE Forums Newbie
 
Steps History: Step 1 Only
Posts: 9
Threads: 3
Thanked 1 Time in 1 Post
Reputation: 11
Default

Viral i pick this question from another forum.

Casandra yes i got the same answer when i calculated it
1/32*1/2*1/4
1/32 for second cousins
1/2 for half cousins
1/4 probability of PKU child if both parents are carriers
Yes i think we should include the probability of father being a carrier
Thanks!
Reply With Quote Quick reply to this message
  #5  
Old 08-26-2012
USMLE Forums Master
 
Steps History: 1+CK+CS
Posts: 1,039
Threads: 189
Thanked 557 Times in 318 Posts
Reputation: 567
Default

Quote:
Originally Posted by electricblue View Post
Viral i pick this question from another forum.

Casandra yes i got the same answer when i calculated it
1/32*1/2*1/4
1/32 for second cousins
1/2 for half cousins
1/4 probability of PKU child if both parents are carriers
Yes i think we should include the probability of father being a carrier
Thanks!
why 1/32 for second cousins? and then separately 1/2 for half cousins?
can you explain how and why did you calculate 1/32? thanks
Reply With Quote Quick reply to this message
  #6  
Old 08-26-2012
USMLE Forums Master
 
Steps History: 1+CK+CS
Posts: 1,039
Threads: 189
Thanked 557 Times in 318 Posts
Reputation: 567
Default

is D correct?
Reply With Quote Quick reply to this message
  #7  
Old 08-26-2012
USMLE Forums Newbie
 
Steps History: Step 1 Only
Posts: 9
Threads: 3
Thanked 1 Time in 1 Post
Reputation: 11
Default

D is correct

for siblings 1/2
for first cousins 1/8 (1/2*1/2*1/2)
for second cousins 1/32 (1/8*1/2*1/2)
ans so on...
please Correct me if i am wrong
regarding half cousin i am not sure i was thinking that we should multiply the whole thing with 1/2....
Reply With Quote Quick reply to this message
  #8  
Old 08-26-2012
USMLE Forums Scout
 
Steps History: Not yet
Posts: 72
Threads: 8
Thanked 11 Times in 11 Posts
Reputation: 21
Default

Thanks for the Q or else I would not have known the concept of half cousins!!

Reply With Quote Quick reply to this message
  #9  
Old 08-26-2012
USMLE Forums Scout
 
Steps History: Step 1 Only
Posts: 15
Threads: 2
Thanked 3 Times in 3 Posts
Reputation: 13
Default

What would have been the answer if they were
1. half cousins
2. Second cousins.

Thanks
Reply With Quote Quick reply to this message
  #10  
Old 08-26-2012
sruthi's Avatar
USMLE Forums Addict
 
Steps History: Not yet
Posts: 109
Threads: 24
Thanked 19 Times in 11 Posts
Reputation: 29
Default

i dnt get wat u mean by HALF cousins!! cud sm1 explain d relashionship!!
Reply With Quote Quick reply to this message
  #11  
Old 08-26-2012
koolkiller88's Avatar
USMLE Forums Master
 
Steps History: Step 1 Only
Posts: 864
Threads: 62
Thanked 370 Times in 224 Posts
Reputation: 380
Default cousin

second cousin-1/8x 1/2x 1/2
Husband being carrier- 1/2
Affected child of carrier parents- 1/4

=1/256
__________________
One day i will infect these microbes and they will search for "Anti-MEbiotic"....
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Quick reply to this message
  #12  
Old 08-26-2012
USMLE Forums Veteran
 
Steps History: 1+CK+CS+3
Posts: 205
Threads: 6
Thanked 151 Times in 80 Posts
Reputation: 161
Default

Quote:
Originally Posted by Casandra View Post
I think it's D 1/256 but it's based on many assumptions so really not sure...

my calculations:
1/2 x 1/4 x (1/8 x 1/2 x 1/2) = 1/256

1/2 - probability that the husband is a carrier (we have to include that, right??? I mean I assume that he's not affected since the stem doesn't say anything and if he's healthy then they will not have a child with PKU but not sure...)
1/4 - probability that, provided both parents are carriers, they will have a child with PKU

1/8 - pool of genes their grandaparents shared (half first cousins)
x 1/2 x 1/2 - portion of genes passed on through 2 generations (from grandparents to the present couple)
chance that 2 related half cousins have same disease is 1/2 x1/2x1/2X1/2=1/16
chance that two heterozygotes will produce a homozygous affected child is in1/4
total probability of these events happening together is 1/16x1/4=1/64

please casandra /anyone else correct me as i did a similar question in KLN in population genetics but now i am not getting how the answer turns out to be 1/256
Reply With Quote Quick reply to this message
  #13  
Old 08-26-2012
USMLE Forums Guru
 
Steps History: 1 + CK
Posts: 454
Threads: 20
Thanked 338 Times in 160 Posts
Reputation: 358
Default

I agree with offpiste .. That's how I worked it out but half second cousin I dont get


Sent from my iPad using Tapatalk HD
__________________
262/261/PASS 1st attempt/238- USCE 1.5 mo (Uni hospital). YOG -2013; home country resident for 6 mo
Reply With Quote Quick reply to this message
  #14  
Old 08-26-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

1/ 32 for second cousin
1/2 for half cousin
1/4 for child being diseased...

1/256

Half first cousins are , cousins who share a single grand parent ..
similarly 1/2 second cousins share single great grand parent....
Reply With Quote Quick reply to this message
  #15  
Old 08-26-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

Quote:
Originally Posted by electricblue View Post
Viral i pick this question from another forum.

Casandra yes i got the same answer when i calculated it
1/32*1/2*1/4
1/32 for second cousins
1/2 for half cousins
1/4 probability of PKU child if both parents are carriers
Yes i think we should include the probability of father being a carrier
Thanks!
From your calculation we also include probability of father being carrier answer would be 1/256 x 1/2 ...
I didnt understand why you said "Yes i think we should include the probability of father being a carrier" ??
Reply With Quote Quick reply to this message



  #16  
Old 08-26-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

Quote:
Originally Posted by Casandra View Post
I think it's D 1/256 but it's based on many assumptions so really not sure...

my calculations:
1/2 x 1/4 x (1/8 x 1/2 x 1/2) = 1/256

1/2 - probability that the husband is a carrier (we have to include that, right??? I mean I assume that he's not affected since the stem doesn't say anything and if he's healthy then they will not have a child with PKU but not sure...)
1/4 - probability that, provided both parents are carriers, they will have a child with PKU

1/8 - pool of genes their grandaparents shared (half first cousins)
x 1/2 x 1/2 - portion of genes passed on through 2 generations (from grandparents to the present couple)

Do you think we should take probability of husband being carrier as 1/2 ?? The ( 1/8x 1/2x1/2) factor that you have used itself is the probability of husband sharing the same gene as his carrier wife..... Am I wrong ?
Reply With Quote Quick reply to this message
  #17  
Old 08-26-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

Quote:
Originally Posted by offpiste View Post
chance that 2 related half cousins have same disease is 1/2 x1/2x1/2X1/2=1/16
chance that two heterozygotes will produce a homozygous affected child is in1/4
total probability of these events happening together is 1/16x1/4=1/64

please casandra /anyone else correct me as i did a similar question in KLN in population genetics but now i am not getting how the answer turns out to be 1/256
your calculation is right but you missed the point that question asked for half second cousins , You have calculated half first cousins ,

.. so the factor 1/16 which u got should be 1/ 64 Hope that helps ,, thanks
Reply With Quote Quick reply to this message
The above post was thanked by:
offpiste (08-26-2012)
  #18  
Old 08-26-2012
USMLE Forums Master
 
Steps History: 1+CK+CS
Posts: 1,039
Threads: 189
Thanked 557 Times in 318 Posts
Reputation: 567
Default

Quote:
Originally Posted by dr.nitowl View Post
Do you think we should take probability of husband being carrier as 1/2 ?? The ( 1/8x 1/2x1/2) factor that you have used itself is the probability of husband sharing the same gene as his carrier wife..... Am I wrong ?
1/8x 1/2x1/2 - is a calculation that they share common genes (it doesn't mean that he's a carrier, he can share those genes and be perfectly healthy = noncarrier)
1/2 - is a chance of him being a heterozygous carrier of autosomal recessive disease
Reply With Quote Quick reply to this message
The above post was thanked by:
dr.nitowl (08-27-2012)
  #19  
Old 08-27-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

Quote:
Originally Posted by Casandra View Post
1/8x 1/2x1/2 - is a calculation that they share common genes (it doesn't mean that he's a carrier, he can share those genes and be perfectly healthy = noncarrier)
1/2 - is a chance of him being a heterozygous carrier of autosomal recessive disease
oh ! thanks.. then how can we account for the half cousin thing??
Reply With Quote Quick reply to this message



Reply

Tags
Genetics-, Step-1-Questions

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

Favorite USMLE Books
What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
fragile X ques doctorF USMLE Step 1 Forum 13 09-16-2016 07:51 PM
step2ck score of 224/96... the big ques as always.. is it enough to sneak in..? drrsahuja USMLE Step 2 CK Forum 7 11-16-2014 02:16 PM
nbme 12 behav science ques. Thrax_usmle USMLE Step 1 Forum 0 03-17-2013 10:03 AM
Biostats ques koolkiller88 USMLE Step 2 CK Forum 5 03-05-2013 07:09 PM
pharm.ques eshaan USMLE Step 1 Forum 7 11-26-2012 06:23 AM

RSS Feed
Find Us on Facebook
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)