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Old 05-03-2011
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Genetics 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..
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Old 05-03-2011
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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.
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Old 05-03-2011
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Quote:
Originally Posted by aries View Post
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
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Old 05-03-2011
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Quote:
Originally Posted by aries View Post
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.
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The above post was thanked by:
nevillenic (05-03-2011), tinhi (05-17-2011)
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Old 05-04-2011
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thank you guys!
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