Calculating Probability of Sickle Cell Disease for Karen & Steve's Child

[gravenewworld]

1. Karen and Steve each have a sibling w/ sickle-cell disease. Neither Karen nor Steve nor any of their parents have the disease, and none of them have been tested to reveal sickle-cell trait. Based on this incomplete information, calculate the probability that if this couple has a child, the child will have sickle cell disease.



1st off, I am not a student, but am just studying on my own to take the MCAT. I am not doing this for any class. The answer is 1/9, but I can not figure this one out. My reasoning thus far:

-So let S be dominant normal allele and let s be the sickle cell gene, which is a recessive trait.

-If Karen and Steve both have a sibling with sickle cell disease, then that means that both parents have to be carriers of the trait since it is a recessive gene. Thus if both parents of both Steven and Karen are carriers, then probability that Karen and Steve are carriers can be calculated from a SsxSs cross. That means that each of them has 1/2 chance of being a carrier.


-Since Karen and Steve don't have sickle cell, then the only way their kids can have sickle cell is if they have the Ss genotype. If they are both carriers, then they have 1/4 probability of having a child with sickle cell.


So the total probability of them having a child with sickle cell should be = Probability that Karen is a carrier x Probability that Steve is a carrier x Probability that they have a kid with sickle cell= 1/2 x 1/2 x 1/4 = 1/16.

I really don't see how 1/9 is the answer. Help please.

 

[Ygggdrasil]

-If Karen and Steve both have a sibling with sickle cell disease, then that means that both parents have to be carriers of the trait since it is a recessive gene. Thus if both parents of both Steven and Karen are carriers, then probability that Karen and Steve are carriers can be calculated from a SsxSs cross. That means that each of them has 1/2 chance of being a carrier.

Incorrect. Try drawing out the Punnet square from a Ss x Ss cross and eliminating any possibilities you know to be wrong (i.e. because neither Karen nor Steve has the disease we know that the are not ss).

 

[gravenewworld]

Alright thanks, Yggg. I see it now. Can't believe I missed this one.