Why 2 in 3 Probability in this mother? - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums Why 2 in 3 Probability in this mother?
 Register FAQs Members List Search Today's Posts Mark Forums Read

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

#1
05-03-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 7 Threads: 4 Thanked 0 Times in 0 Posts Reputation: 10
Why 2 in 3 Probability in this mother?

Husband has cystic fibrosis, mother's sister has cystic fibrosis..chances that child will have the disease? It says 1/3 can someone plz explain the calculation how come mother's probability that she is a carrier is 2/3?

It's a USMLE World Q..

#2
05-03-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 239 Times in 144 Posts Reputation: 249

probability of passing disease to the child..1/2
probablity of mother being a carrier.2/3..(as one of her sister has the disease and out of the remaining 3 siblings two will be carrier and one will be normal)

Chance of child getting the disease..1/2 x 2/3 =1/3

Hope i am correct.
#3
05-03-2011
 USMLE Forums Addict Steps History: 1+CK+CS+3 Posts: 117 Threads: 24 Thanked 47 Times in 24 Posts Reputation: 57

Quote:
 Originally Posted by aries husband has cystic fibrosis,mothers sister has cystic fibrosis..chances that child will have the disease? it says 1/3 can someone plz explain the calculation how come mothers probability that she is a carrier is 2/3? its a usmle world q..

if mother cousin is cystic fibrosis means mothers parents are carrier , from a punnet square taking mothers parents as carrier , propability of mother getting disease is 1/4 [25 percent] but don't have .so mother being carrier according to normal punnet is 2/4, here 4 includes all probabilities, but we already know she is not disease so remove one from 4 ie 3 , so chances of mother being carrier is 2/3

#4
05-03-2011
 USMLE Forums Master Steps History: Not yet Posts: 557 Threads: 69 Thanked 1,049 Times in 351 Posts Reputation: 1069

Quote:
 Originally Posted by aries husband has cystic fibrosis,mothers sister has cystic fibrosis..chances that child will have the disease? it says 1/3 can someone plz explain the calculation how come mothers probability that she is a carrier is 2/3? its a usmle world q..
CF is autosomal recessive, so
2. Mom is not rr since she does not have the disease. That leaves 3 possibilities: she is RR, Rr, Rr. Note that in 2 out of the 3 remaining possibilities she is a carrier, hence the 2/3.

For sure Dad will pass r to the kid, so the kid will either be a carrier or have the disease depending on Mom.
If Mom is a carrier, probability she will pass the trait is 1/2.
Since 2/3 probability she is a carrier, 2/3 x 1/2 = 1/3

Quote:
 probablity of mother being a carrier.2/3..(as one of her sister has the disease and out of the remaining 3 siblings two will be carrier and one will be normal)
@ struggle, nice reasoning, except for this with the Mom. Remember that each person's probability of inheriting a trait is independent, so it doesn't necessarily means that 2 siblings will be carrier and one normal (they can all be normal or can all have the disease). This is just used to calculate the probability, so each sibling's risk need to be calculated independently.
 The above post was thanked by: nevillenic (05-03-2011), tinhi (05-17-2011)
#5
05-04-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 7 Threads: 4 Thanked 0 Times in 0 Posts Reputation: 10

thank you guys!

 Tags Genetics-, Step-1-Questions

Message:
Options

## Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
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.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
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.