uworld genetics question help (probabilities)

[axeon123]

---

Members don't see this ad.

Hey guys - was wondering if anyone can help me with some genetics on step1. The problem question is basically this

1. dealing with hemophilia, so X-linked recessive
2. parents are not affected, but the son is affected
---> mother is the carrier
---> son picked up the bad X chromosome
3. sister is not affected (so could be carrier or homozygous normal)

They ask to calculate the probability that the sister will have an affected child (sex of the child is unknown). And I'm having a little bit of trouble figuring this out because I thought it also depends on who the father is, because they do not say if the father is affected.

They end up doing some weird probability
1. p(sister is carrier) = 0.5 - this makes sense, 50/50 inheritance from mother
2. p(sister will have male child) = 0.5 - also makes sense, 50/50 inheritance from the father
3. p(offspring will inherit bad x chromosome) = 0.5 - make sense

But, do you not have to take into account the father? I think I'm missing something here...

 

[username456789]

Off the top of my head, I would assume that since the disease frequency (and thus the carrier frequency among males) is so rare (I don't remember what it is, but it's definitely below 1/100), the male's genotype can be excluded, since the calculation would result in such a minuscule change.

In other words, assume the potential father is unaffected, because it's by far the most common situation. If you carried out the calculation using the disease/carrier frequency, it might change the overall answer by 0.000000001% or something.

 

[StIGMA]

You assume the potential father is not affected (the rate of affected males is so low in the population that it will not affect your calculation). Basically, only assume an affected father if it is explicitly stated in this situation.

So the probability that the sister will have an affected child is the chance she is a carrier (0.5) x the chance of a male (.5) x chance of the bad sex chromosome = 0.125.

Think about if she is a carrier... then there is a 1/4 chance that each potential offspring is affected (0/2 females and 1/2 males). Multiply this by 1/2 for the odds that she is a carrier or not.

 

[hartbot]

Hey guys - was wondering if anyone can help me with some genetics on step1. The problem question is basically this

1. dealing with hemophilia, so X-linked recessive
2. parents are not affected, but the son is affected
---> mother is the carrier
---> son picked up the bad X chromosome
3. sister is not affected (so could be carrier or homozygous normal)

They ask to calculate the probability that the sister will have an affected child (sex of the child is unknown). And I'm having a little bit of trouble figuring this out because I thought it also depends on who the father is, because they do not say if the father is affected.

They end up doing some weird probability
1. p(sister is carrier) = 0.5 - this makes sense, 50/50 inheritance from mother
2. p(sister will have male child) = 0.5 - also makes sense, 50/50 inheritance from the father
3. p(offspring will inherit bad x chromosome) = 0.5 - make sense

But, do you not have to take into account the father? I think I'm missing something here...

I think youre supposed to assume the person theyre mating with is never affected. So if this is the case, then the only way this lady can have an affected child is if she gives a mutated x-gene to a son. The chance of having a son is 0.5. You know she cant have an affected daughter because the daughter would receive a normal x from the unaffected dad, which would block out the adverse affects from mommy's bad x.

You take all those 0.5s and times them together and get 1/8, since all these condiitons need to be met for her to have a hemophiliac son

Beat me to it.