question 114 CBT3

[Mtrader]

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This is question 114 CBT3

The parents are Pp what is the probability that the oldest sib is a heterozygote for the PKU trait?



A. 1/4
B. 1/2
C. 1/3
D. 2/3

answer:
D is the best answer. Remember, by conventional generic pedigree notation, the oldest sib is the one located at II-1. She does not show the phenotype for PKU. Logic tells us that both parents must be Pp and that any unaffected offspring of theirs each has a 2/3 chance of being Pp, which denotes that one is heterozygous for this trait. The best answer is D.

i dont get how any PpxPp doesnt lead to 1/2 being heterozygous this way i see it is
PpxPp= PP, Pp, Pp, pp
i must be missing somthing.

Related to this is #113 the probablities are really necessary to answer the question but at the end it says:

"We are told PKU is inherited as an autosomal recessive disease, meaning that individuals who have this disease have the genotype pp....

....She has a 2/3 chance of being Pp and a 1/3 chance of being PP..."

i dont understand where they are getting these fractions. can anyone help?

 

[Jepstein30]

This is question 114 CBT3

The parents are Pp what is the probability that the oldest sib is a heterozygote for the PKU trait?



A. 1/4
B. 1/2
C. 1/3
D. 2/3

answer:
D is the best answer. Remember, by conventional generic pedigree notation, the oldest sib is the one located at II-1. She does not show the phenotype for PKU. Logic tells us that both parents must be Pp and that any unaffected offspring of theirs each has a 2/3 chance of being Pp, which denotes that one is heterozygous for this trait. The best answer is D.

i dont get how any PpxPp doesnt lead to 1/2 being heterozygous this way i see it is
PpxPp= PP, Pp, Pp, pp
i must be missing somthing.

Related to this is #113 the probablities are really necessary to answer the question but at the end it says:

"We are told PKU is inherited as an autosomal recessive disease, meaning that individuals who have this disease have the genotype pp....

....She has a 2/3 chance of being Pp and a 1/3 chance of being PP..."

i dont understand where they are getting these fractions. can anyone help?

Pp x Pp = 1/4 PP 1/2 Pp 1/4 pp

Pedigree shows the oldest sibling isn't pp (no disease) so must be PP or Pp. Using the odds above, we adjust the probabilities to 1/3 PP 2/3 Pp (Pp is 2x likely as PP).

You only took the parental genotypes into account when the question provided a family pedigree with more useful information.

They know the sibling doesn't have the disease so no reason to account for pp in the odds.

 

[Mtrader]

Pp x Pp = 1/4 PP 1/2 Pp 1/4 pp

Pedigree shows the oldest sibling isn't pp (no disease) so must be PP or Pp. Using the odds above, we adjust the probabilities to 1/3 PP 2/3 Pp (Pp is 2x likely as PP).

You only took the parental genotypes into account when the question provided a family pedigree with more useful information.

They know the sibling doesn't have the disease so no reason to account for pp in the odds.

thanks! i understand now. So to answer this question the term"based on the pedigree shown" is the key. i should have eliminated pp from my calculations "based on the figure" Is this correct?

 

[Jepstein30]

thanks! i understand now. So to answer this question the term"based on the pedigree shown" is the key. i should have eliminated pp from my calculations "based on the figure" Is this correct?

Yep! If you weren't shown the pedigree, you have the correct answer. But with the additional info provided by the pedigree, we can 'zone' in on the odds.