Step I Phenylketonuria genetics question

[arda]

Advertisement - Members don't see this ad

What is the probability that the asymptomatic brother or sister of a child with the disease is a heterozygous carrier?

Answer is 2/3.Can somebody explain it ? parents can be either both carrier,both sick or either one of them is sick or carier right?

 

[Missorleans]

What is the probability that the asymptomatic brother or sister of a child with the disease is a heterozygous carrier?

Answer is 2/3.Can somebody explain it ? parents can be either both carrier,both sick or either one of them is sick or carier right?

It is possible that one of the parents could have PKU. PKU patients do live long lives now, they should tell you that neither parent has PKU.

But going with the most likely scenario, two heterozygous parents, do you understand how you get 2/3?

 

[AnalisCanalis]

With these questions you just have to go with the best(the most likely) answer, since it's impossible to know the genetic makeup of the parents without them telling you.

Assuming two heterozygous parents, 2/4 children will be heterozygous. Though, in this particular case we already know that the patients sibling is not homozygous for PKU(since the sibling is asymptomatic) leaving us with 2/3 probability for heterozygosity and 1/3 probability that the brother is a non-carrier.

 

[arda]

Then we are assuming that parents are both carrier right ?

 

[scoKraz4]

Shouldn't you always assume the parents are heterozygous when dealing with recessive diseases unless otherwise stated.

 

[Psai]

Then we are assuming that parents are both carrier right ?

No need to assume, they are carriers for sure

 

[arda]

Why ? one of them can be sick and other one can be a carrier,we are excluding this possibility.
I will list other possbilities that come to my mind

A :healty
a: recesive gene

F:Aa M:Aa for this couple to have heterozygous child is 2/4 and having a carier sibling is 2/3
F:AA M:aa for this one 1/1 but no sibling has disease
F:aa M:aa for this one zero,every child will have the disease
F:Aa M:aa for this one 1/2 , and having a carrier sibling is 1/1
F:AA M:AA for this one zero,no child will be sick or carrier
F:aa M:AA for this one 1/1,but no child will have the disease

Parents can be either of these listed above,and answers have both zero,2/3, 1/2 and 1/1.

I am just thinking it as if it is a probability question what am i missing ? am i overthinking ?

 

[EazyE1907]

Why ? one of them can be sick and other one can be a carrier,we are excluding this possibility.
I will list other possbilities that come to my mind

A :healty
a: recesive gene

F:Aa M:Aa for this couple to have heterozygous child is 2/4 and having a carier sibling is 2/3
F:AA M:aa for this one 1/1 but no sibling has disease
F:aa M:aa for this one zero,every child will have the disease
F:Aa M:aa for this one 1/2 , and having a carrier sibling is 1/1
F:AA M:AA for this one zero,no child will be sick or carrier
F:aa M:AA for this one 1/1,but no child will have the disease

Parents can be either of these listed above,and answers have both zero,2/3, 1/2 and 1/1.

I am just thinking it as if it is a probability question what am i missing ? am i overthinking ?

waaaaaaaaaaaaay overthinking it.

because

Shouldn't you always assume the parents are heterozygous when dealing with recessive diseases unless otherwise stated.

 

[mcloaf]

I, too, was taught to always assume that anyone not specifically noted as diseased was considered healthy in genetics problems (assuming it isn't one of those annoying Bayesian problems involving allele frequency). Mom and dad must be carriers since they aren't described as ill and they have an affected child.