TBR CBT #3, question 116

[happyfellow]

Question #116 deals with the probability of an offspring being heterzygous for the PKU trait (autosomal recessive disease). We already know that both the parents are heterozygous and therefore I answered 1/2 probability that the offspring would also be hetero. If we cross Pp X Pp, we get 1/2 hetero....right? However, the answer is 2/3. How can we get a 2/3 probability when there are 4 possibilities? Thanks in advance

-happy

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[fizzgig]

i don't have TBR but i bet they wanted to know the odds that a NON expressing individual was carrying - so eliminate the pp and you're left with PP Pp Pp - if your kid does not express the trait, there's a 2/3 chance that s/he's carrying.

 

[Stephen2009]

#1: First, the question states "based on the pedigree", thus we know the oldest child is NOT affected with PKU.

#2: with this info, we only look at the unaffected offsprings which is
PP, Pp, and Pp (ignoring pp)

#3: 2 heterozygotes out of 3 possibilities is 2/3, your answer