Genetics problem on carrier status of Cystic Fibrosis

[Bathrover]

Ben and Jill who are both 30 years old want to have a baby.
Jill’s sister has a child affected with cystic fibrosis.
What is the risk that Jill is a carrier for the disease allele?

The answer given is 50%.

I understand CF is a AR disease. However how do we figure out Jill's parent's genotype to draw a punnet square and figure out her chance of being a carrier. (All we really know is her sister has an affected child so most likely her sister is a carrier... from this how do we extrapolate?)

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[sinombre]

edit: misread the question.

 

[SN12357]

Because the sister is a carrier, at least one of Jill's parents must also be a carrier. Generally you then assume normal population risk (fairly low) for the other parent. So Jill's risk is essentially (although not exactly, depending on how nitpicky your instructor is) 50% as she had a 50% chance of inheriting the recessive allele from the carrier parent.

In these genetic problems you often have to work backward from known carriers to figure out who the carriers must have been in previous generations.

 

[Bathrover]

I thought we could ask questions that relate to step exam (albeit remotely)... =)

Assuming the most probable genotype for the parents I did conclude it would be AAxAa resulting in 50% chance, however if Jill had any affected siblings we would assume the parents to be AaxAa which would yield 2/3 risk (assuming Jill is unaffected, which appears to be the case since she is already 30) or 50% if unknown affected state.

The problem I have with this type of question is we assume the most probable genotype for the parents so the answer can never 100% be assumed as Jill having 50% risk to being a carrier... Parents could have been AaxAa (Parents could both be carriers, more rare) or AAxaa (Jill's mother had CF but reproduced with a viable male, even more rare but still possible).

PS: I guess my question has more to do with genetic pedigrees and making assumptions, for examination questions we would assume the most probable genotypes based on the normal prevalence rates for the given disease?

 

[SouthernSurgeon]

SDN is not for homework

 

[sinombre]

Assuming the most probable genotype for the parents I did conclude it would be AAxAa resulting in 50% chance, however if Jill had any affected siblings we would assume the parents to be AaxAa which would yield 2/3 risk (assuming Jill is unaffected, which appears to be the case since she is already 30) or 50% if unknown affected state.

Right, but she doesn't. She has a sister who has a child with CF, not a sister with CF. Meaning you should assume one parent is a carrier. So the parents are Aa and AA.

 

[Bathrover]

Where is the section we can ask questions regarding step I examination... I am pretty sure they had this unless things have changed =/

 

[Ismet]

Where is the section we can ask questions regarding step I examination... I am pretty sure they had this unless things have changed =/

In the Step 1 forum...

...moving to Step 1 forum.

 

[demayette]

Right, but she doesn't. She has a sister who has a child with CF, not a sister with CF. Meaning you should assume one parent is a carrier. So the parents are Aa and AA.

So why can't one assume that parents are Aa and (Aa or AA)?

 

[sinombre]

So why can't one assume that parents are Aa and (Aa or AA)?

Because the gene frequency is low. Unless you have a reason to, you don't assume a random person has a mutant allele. Statistically there is some chance that both parents could be carriers instead of just one, but the odds are low enough that you can ignore that possibility.

Sometimes problems will give you a gene frequency (e.g. 1% of the population is a carrier) in which case you would take that in to consideration. But not in this case.