What is the probability that the child is affected?

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In summary: The probability of inheritance of a mutant allele is 1/2 from I-1 and 1 from I-2. The probability of II-3 being a carrier is 1/2. OK, but one carrier and one not carrier don't have the same odds as two carriers. Hint: what does it mean that the gene is recessive? How do recessive genes manifest?
  • #1
TytoAlba95
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Homework Statement


The following pedigree shows that the trait is autosomal recessive. The probability of carriers in the population is 1/3. What is the probability that the child of III-3 and 4 is affected?
X4pkM.png

2. Relevant info
The answer provided is 1/36.

The Attempt at a Solution


The probability of II-3 being a carrier is 1/2 and that of II-4 is 1/3. The probability of two carriers having an affected child is 1/2. So the probability that II-3 and II-4 are carriers and their child is affected is 1/2 x1/3 x1/2=1/12.

(Edited to remove typo.)
 

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  • #2
Hm. There seems to be a problem with II versus III in your solution.
II is the middle gen. III is the last gen.

What is the probability of II3's inheritance from one I1? What is the probability from I2?
What is the total?
 
  • #3
DaveC426913 said:
What is the probability of II3's inheritance from one I1? What is the probability from I2?
What is the total?

The probability of inheritance of a mutant allele is 1/2 from I-1 and 1 from I-2. The probability of II-3 being a carrier is 1/2.
 
  • #4
OK, but one carrier and one not carrier don't have the same odds as two carriers.
Hint: what does it mean that the gene is recessive?
How do recessive genes manifest?
 
  • #5
DaveC426913 said:
OK, but one carrier and one not carrier don't have the same odds as two carriers.
Hint: what does it mean that the gene is recessive?
How do recessive genes manifest?
Recessive alleles manifest only when they occur in a pair. To be honest I'm not able to follow your indication.
 
Last edited:
  • #6
SanjuktaGhosh said:
The probability of II-3 being a carrier is 1/2
This is incorrect. It may be helpful to try writing out the genotypes of the parents (I1 and I2), then trying to infer what the genotype of II3 must be.

SanjuktaGhosh said:
The probability of two carriers having an affected child is 1/2
This is incorrect. Try drawing a Punnet square of the cross.

SanjuktaGhosh said:
The answer provided is 1/36.
When I analyze the problem, I do not get this answer.
 
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1. What factors determine the probability that a child is affected?

The probability of a child being affected by a particular condition or disease is influenced by a variety of factors, including genetics, family history, environmental factors, and lifestyle choices.

2. Is there a way to accurately predict the probability of a child being affected?

While there are some methods of predicting the likelihood of a child being affected, such as genetic testing, it is not always possible to accurately determine the probability of a child being affected as there are many variables at play.

3. How does the probability of a child being affected change with each additional risk factor?

This can vary depending on the specific condition or disease in question, but in general, the more risk factors a child has, the higher their probability of being affected becomes. However, there are cases where a single risk factor may significantly increase the likelihood of being affected.

4. Can the probability of a child being affected change over time?

Yes, the probability of a child being affected can change over time as they grow and are exposed to different factors. This is especially true for conditions or diseases with a genetic component, as genetic expression can change over time.

5. Is there a way to lower the probability of a child being affected?

In some cases, there may be ways to reduce the likelihood of a child being affected by a condition or disease. This can include lifestyle changes, preventative measures, and early detection and treatment. However, it is important to consult with a healthcare professional for personalized recommendations.

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