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#1
05-21-2014
 USMLE Forums Scout Steps History: Not yet Posts: 98 Threads: 21 Thanked 36 Times in 26 Posts Reputation: 46
genetics

This is a question from kaplan videos but i cannot understand why this is the answer.
You know father is a carrier for CF, mother has 1/10 possibility being a carrier. What is the possibility the baby being a carrier?
1/10 * 1/4= 1/40
I cannot understand why it is 1/4 and not 1/2

#2
05-21-2014
 USMLE Forums Newbie Steps History: 1 + CK Posts: 6 Threads: 2 Thanked 3 Times in 3 Posts Reputation: 13

Because CF is autosomal recessive.
 The above post was thanked by: usmlerun (05-21-2014)
#3
05-21-2014
 USMLE Forums Newbie Steps History: Not yet Posts: 5 Threads: 3 Thanked 2 Times in 2 Posts Reputation: 12

Wait.. it asks what the chances are that the baby is a CARRIER (not if he has the disease)

The father is a carrier. So he has both dominant and recessive alleles. The chances that he passes on the recessive allele should be 1/2.

To this you ADD the probability that the mother could pass it on to the child which would be negligible. She has 1/10 chance she is a carrier and IF she is 1/2 chance she passes it onto her child. So her chances are 1/10*1/2=1/20.

So the answer to the question would be 11/20.

Now if the question asked what are the chances that the child has the DISEASE you would have to think like this:

1/2 chance that the father passes it on
1/2 chance that the mother passes it on IF she is a carrier
Chances that she is a carrier are 1/10
All of the above would have to occur for their child to have the disease.

So you would multiply all of these together.

1/2*1/2*1/10=1/40
 The above post was thanked by: usmlerun (05-21-2014)

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