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

In summary, if Karen and Steve have a child, the child has a 1/16 probability of having sickle cell disease.
  • #1
gravenewworld
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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.
 
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  • #2
gravenewworld said:
-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).
 
  • #3
Alright thanks, Yggg. I see it now. Can't believe I missed this one.
 

1. What is the likelihood of Karen and Steve's child having sickle cell disease?

The likelihood of Karen and Steve's child having sickle cell disease depends on several factors, including the genetic makeup of the parents and the inheritance pattern of the disease. Sickle cell disease is an inherited disorder, meaning it is passed down from parents to their children. If both parents are carriers of the sickle cell gene, there is a 25% chance that their child will have sickle cell disease.

2. How is the probability of sickle cell disease calculated?

The probability of sickle cell disease is calculated by looking at the inheritance pattern of the disease. In this case, Karen and Steve's child has a 25% chance of having sickle cell disease if both parents are carriers of the sickle cell gene. This probability can be calculated using Punnett squares or through genetic testing.

3. What is the role of genetics in determining the probability of sickle cell disease?

Genetics play a major role in determining the probability of sickle cell disease. The disease is caused by a mutation in the gene that codes for hemoglobin, the protein in red blood cells that carries oxygen. If a person inherits two copies of the mutated gene (one from each parent), they will have sickle cell disease. If they inherit only one copy of the mutated gene, they will be a carrier of the disease.

4. Are there any other factors that may affect the probability of sickle cell disease for Karen and Steve's child?

Aside from genetics, there are no other factors that significantly affect the probability of sickle cell disease for Karen and Steve's child. However, it is important to note that the probability is a prediction and not a guarantee. Even if both parents are carriers of the sickle cell gene, their child may not necessarily have the disease.

5. Can the probability of sickle cell disease change over time?

The probability of sickle cell disease for Karen and Steve's child will not change over time. Once the child is born, their genetic makeup is set and cannot be altered. However, as the child grows and develops, they may undergo genetic testing to confirm whether or not they have sickle cell disease. This testing can provide a more accurate determination of the child's probability of having the disease.

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