What is the probability that the child is affected?

[TytoAlba95]

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?

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.)

 

[DaveC426913]

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?

 

[TytoAlba95]

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.

 

[DaveC426913]

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?

 

[TytoAlba95]

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.

 

[Ygggdrasil]

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.

The probability of two carriers having an affected child is 1/2

This is incorrect. Try drawing a Punnet square of the cross.

The answer provided is 1/36.

When I analyze the problem, I do not get this answer.