What is the probability that the child is affected?

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SUMMARY

The probability that the child of individuals III-3 and III-4 is affected by an autosomal recessive trait is calculated to be 1/36. The analysis involves determining the carrier status of the parents, with II-3 having a 1/2 chance of being a carrier and II-4 a 1/3 chance. The probability of two carriers producing an affected child is 1/4, leading to the final calculation of 1/2 x 1/3 x 1/4 = 1/24, which indicates a misunderstanding in the initial calculations. The correct approach requires careful consideration of the inheritance patterns and the use of a Punnett square for clarity.

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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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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?
 
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.
 
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?
 
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:
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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