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#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 236 Times in 143 Posts Reputation: 246

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: 116 Threads: 23 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
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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
1. Dad is rr
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

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