Genetic probability Question - 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 02-19-2010
USMLE Forums Scout
 
Steps History: Not yet
Posts: 33
Threads: 30
Thanked 55 Times in 22 Posts
Reputation: 65
Genetics Genetic probability Question

Prevalence of homozygotes in a population is 1%. If a woman is a known carrier marries somebody from that population then what is the probability that they'll produce an affected homozygous child?

A- 0.09
B- 0.07
C- 0.05
D- 0.025
E- 0.25
F- 0.50
G- 0.10
Reply With Quote Quick reply to this message
The above post was thanked by:
asmz34 (10-09-2013), danylo (10-14-2013), drzamzam (07-16-2015), radwan5711 (01-30-2011), Taiwan_Guy (05-08-2011), Usmle16Forall (09-21-2016)



  #2  
Old 02-19-2010
antanaRK's Avatar
USMLE Forums Scout
 
Steps History: 1+CK+CS
Posts: 18
Threads: 1
Thanked 60 Times in 11 Posts
Default

Anybody knows the answer to that
Reply With Quote Quick reply to this message
  #3  
Old 02-19-2010
USMLE Forums Scout
 
Steps History: Step 1 Only
Posts: 42
Threads: 17
Thanked 1 Time in 1 Post
Reputation: 11
Default

i think the answer is E
Reply With Quote Quick reply to this message
 
  #4  
Old 02-19-2010
antanaRK's Avatar
USMLE Forums Scout
 
Steps History: 1+CK+CS
Posts: 18
Threads: 1
Thanked 60 Times in 11 Posts
Default

Quote:
Originally Posted by Abdi View Post
i think the answer is E
Please tell me how did u arrive to this conclusion!
Reply With Quote Quick reply to this message
  #5  
Old 02-19-2010
Breex's Avatar
USMLE Forums Scout
 
Steps History: 1+CK+CS
Posts: 51
Threads: 3
Thanked 27 Times in 17 Posts
Reputation: 37
Default

The woman has three possibilities:
married to affected homozygous 1%
married to affected heterozygous (should be calculated by Hardy weinburg)
married to homozygous normal person (the remaining possibility)

These probabilities should be combined

If some one can do the math for us to know the answer
Reply With Quote Quick reply to this message
  #6  
Old 06-07-2010
USMLE Forums Scout
 
Steps History: Not yet
Posts: 51
Threads: 2
Thanked 32 Times in 14 Posts
Reputation: 42
Default

I know this has been up for a while and has not been answered. I would like to know what you guys think...

I think the answer is C
Reply With Quote Quick reply to this message
  #7  
Old 06-07-2010
USMLE Forums Guru
 
Steps History: ---
Posts: 487
Threads: 69
Thanked 296 Times in 133 Posts
Reputation: 316
Default C

its C 0.05
disease prevalence 1% = 1/100 = q2
allele frequency = q = 1/10
when disease frequency is less then 1/100 p(normal allele frequency) is considered 1
so....carrier frequency will be 2pq = 2q = 1/5
for AR disease , father must be carrier to transmit disease to child...
probability that women will marry carrier man is 1/5
probability that women will transmit her diseased allele to child is 1/2
probability that man will transmit his diseased allele to child is 1/2
now just multiply 1/5*1/2*1/2 = 1/20 = 0.05
Reply With Quote Quick reply to this message
The above post was thanked by:
abeer (10-08-2013), Androide89 (08-29-2010), asmz34 (10-09-2013), danylo (10-14-2013), dr_kg (10-12-2013), drzamzam (07-16-2015), inder (06-07-2010), nsesereso (10-12-2013), Usmle16Forall (09-21-2016), zen228 (04-16-2014)
  #8  
Old 06-07-2010
USMLE Forums Scout
 
Steps History: Not yet
Posts: 51
Threads: 2
Thanked 32 Times in 14 Posts
Reputation: 42
Default

Oh okay sweet.
I did it a bit differently as I included the likelyhood of marrying a homozygous ressessive and having a child (ended up being .005)
I also didn't assume p as being 1 (rather .9) so i got .045 instead of .05
The grand total was .05.
I guess either way works.
Reply With Quote Quick reply to this message
  #9  
Old 10-07-2013
lokya's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 12
Threads: 3
Thanked 2 Times in 2 Posts
Reputation: 12
Default

Quote:
Originally Posted by khushboo View Post
its C 0.05
disease prevalence 1% = 1/100 = q2
allele frequency = q = 1/10
when disease frequency is less then 1/100 p(normal allele frequency) is considered 1
so....carrier frequency will be 2pq = 2q = 1/5
for AR disease , father must be carrier to transmit disease to child...
probability that women will marry carrier man is 1/5
probability that women will transmit her diseased allele to child is 1/2
probability that man will transmit his diseased allele to child is 1/2
now just multiply 1/5*1/2*1/2 = 1/20 = 0.05


I have a doubt..is the probability of man transmitting the dz is 1/4 coz the question says the woman will marry a guy from homozygous population??

Can anybody explain this question again?
Reply With Quote Quick reply to this message
  #10  
Old 10-08-2013
Anwar's Avatar
USMLE Forums Scout
 
Steps History: 1+CK+CS+3
Posts: 97
Threads: 16
Thanked 63 Times in 22 Posts
Reputation: 73
Post my solution

may i suggest another solution

since 1% of that population is homozygous normal or diseased
if the woman marries a homozygous normal from that 1% she wont get a diseased homozygous baby
if she marries a homozygous diseased person from that same 1% she will get a homozygous diseased baby but for our calculation purpose we are going to neglect that homozygous diseased population since it will be negligible as it is an AR disease and it will be a small fraction of that 1%

so the probabability of that woman marrying a heterozygous carrier is 99% or 0.9
the probability of her having a homozygous diseased baby if she marries a heterozygous carrier = 1/4 or 0.25
the probability of husband having a homozygous diseased baby if he marries a heterozygous carrier = 1/4 or 0.25
multiply all the probabilities
0.9 x 0.25 x 0.25 = 0.05
Reply With Quote Quick reply to this message
The above post was thanked by:
danylo (10-14-2013), drHubaishi (11-19-2013)
  #11  
Old 10-09-2013
USMLE Forums Guru
 
Steps History: Not yet
Posts: 339
Threads: 94
Thanked 331 Times in 150 Posts
Reputation: 291
Default

answer is c
Reply With Quote Quick reply to this message
  #12  
Old 10-12-2013
USMLE Forums Newbie
 
Steps History: Not yet
Posts: 2
Threads: 1
Thanked 0 Times in 0 Posts
Reputation: 10
Arrow

I think answer is 0.25or e. since woman gonna merrie man from that 1% population of homozygous , it's 1/2 chance that that man is affected homozygous and woman transfers her affected allele is 1/2, so calculating 1/2 times 1/2=0.25
Reply With Quote Quick reply to this message
  #13  
Old 10-12-2013
USMLE Forums Veteran
 
Steps History: Step 1 Only
Posts: 216
Threads: 23
Thanked 180 Times in 72 Posts
Reputation: 190
Default

The question is asking you this -

Probability of a homozygous child if someone "from the population" marries a known carrier female. Let's call her Miss X.

Now by population, it means 2 possibilities -

Situation 1 - Another carrier from the population marries Miss X. The probability of this event happening is - 2pq = 2x0.9x0.1 = 0.18 (Note: the square of q = 0.01, hence 1=0.1 and p=1-0.1 = 0.9. We can't ignore p in the equation because the prevalence is high.)
Now if this mating occurs, probability of having a homozygous child = 0.25 (draw a punnet square if you please.)
Hence combining the probabilities, final probability from situation 1 = 0.18x0.25 =0.45.

Situation 2 -A homozygous diseased male marries Miss X. Note that the question doesn't suggest that the female will only marry a phenotypically normal guy. The probability of this event happening is 0.01 (frequency of homozygotes is given in the question as 1%).
If this mating occurs, probability of having a homozygous child = 0.5.
Combined probability from situation 2 = 0.05.

Now to calculate the final answer, we must understand that EITHER of the 2 situations would give us our homozygous child. In these scenarios, the probabilities are to be added (not multiplied.)

So combining situation 1 and 2, final probability = 0.45 + 0.05 = 0.5

I know this is confusing and we probably won't be tested such complicated applications but once you read it slowly and refer to your genetics book, you would get it. I think a similar questions can be found in the Kaplan qbank. Hope that helped.
Reply With Quote Quick reply to this message
The above post was thanked by:
docrhi (06-01-2015), drHubaishi (11-19-2013), Pachys (04-04-2016)



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
Child Development and Genetic Disorders geof1 USMLE Step 2 CK Forum 4 04-03-2016 10:03 PM
Probability Calculation anoop_1198 USMLE Step 1 Forum 4 06-07-2010 04:19 AM
Genetic Probabilities? doctorF USMLE Step 1 Forum 12 04-05-2010 09:47 AM
How can i memorize Genetic disorder? Eric Zhuang USMLE Step 2 CK Forum 3 03-01-2010 08:56 AM
Genetic Coding Problem ngnorah USMLE Step 1 Forum 1 01-21-2010 08:12 PM

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)