Tbr fl#3 bs #116

[dougkaye]

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"Based on the pedigree shown in Figure 1, what is the probability that the oldest sib is a heterozygote for the PKU trait?"

This is a Pp x Pp cross where we know the child in question does NOT have the autosomal recessive trait (PKU).

How did they get 2/3 chance? If both parents are heterozygous for an autosomal recessive disease, the chance of the child being heterozygous for trait is 1/2. Just because you can rule out the pp outcome (the child doesn't have the disease and is therefore PP or Pp) doesn't change the actual statistical likelihood of her being Pp. It is still a 1/2 chance, right? Well, according to the key I guess not but I thought I'd check with you more statistically-minded folks out there to get the logic for BR's answer.

 

[attixx]

The question isn't asking what the future probability is, it asked what the current probability is based on the facts. That's how I approached it anyway.

You knock out a 1/4th of the pedigree because the homozygous recessive trait is known to be not there. So of the three remaining choices based on inheritance, what is the probability of heterozygous. The answer 1/2 is implying that there is a 50% chance that the sibling is homozygous and 50% chance of heterozygous. But we know that based on pedigrees, there is a greater chance of heterozygous from the 1:2:1 ratio. Take out the recessive part, and you get 1:2, or 2/3 heterozygous.

 

[Rabolisk]

I flip a regular coin and tell you that it did not land on heads. What is the probability that it landed on tails?

 

[dougkaye]

I flip a regular coin and tell you that it did not land on heads. What is the probability that it landed on tails?

If I flip a regular coin and get a head, and ask "what is the probability of getting a head?", it's still 50%. But I understand what you're both saying. Thanks!

 

[451617]

The question isn't asking what the future probability is, it asked what the current probability is based on the facts. That's how I approached it anyway.

You knock out a 1/4th of the pedigree because the homozygous recessive trait is known to be not there. So of the three remaining choices based on inheritance, what is the probability of heterozygous. The answer 1/2 is implying that there is a 50% chance that the sibling is homozygous and 50% chance of heterozygous. But we know that based on pedigrees, there is a greater chance of heterozygous from the 1:2:1 ratio. Take out the recessive part, and you get 1:2, or 2/3 heterozygous.

thanks!

 

[451617]

this is from the next question # 115 of same cbt and BS:

"A woman who expresses the PKU phenotype marries a man whose father did not carry the PKU gene, but whose mother did express the disease. What is the probability that their child will NOT show any symptoms of PKU?"

i take "expresses the phenotype" to mean that she has the potential to express the disease ie: Pp

and

"expresses the disease" to mean pp.

should i be reading both variations as pp?