genetics question

[BoneMental]

I came across this practice question in my studies and I feel like it is incorrect.

39) Assume the autosomal condition X is caused by having two recessive alleles. If a father is a carrier of condition X and the mother does not have condition X (nor is she a carrier), what is the probability that a daughter of these two parents will be a carrier of a recessive allele?

A)100%
B) 75%
C) 50%
D) 25%

I said D, because there's a fifty percent chance you will have a female child, and a fifty percent chance one of the offspring is a carrier. Thus, (1/2)(1/2) = 1/4 = 25%.

However, the explanation said C was correct because you did not need to take the gender of the offspring into account because this was not a sex-linked trait. WTF? Has anyone heard of treating a problem like this using this logic before? I've always taken the gender into account regardless of whether or not it was sex-linked, given a specific gender was asked for in the question stem...!

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[docelh]

The right answer is 50%.

The question asks the likelihood that the daughter is a carrier Xx. Since the mom is totally healthy, here genes are irrelevant; thus, the law of multiplication doesn't apply.

The carrier state entirely depends on the probability of the dad's recessive gene being in the mix. And that is 50%. A punnett square will bear this out.

 

[BoneMental]

The right answer is 50%.

The question asks the likelihood that the daughter is a carrier Xx. Since the mom is totally healthy, here genes are irrelevant; thus, the law of multiplication doesn't apply.

The carrier state entirely depends on the probability of the dad's recessive gene being in the mix. And that is 50%. A punnett square will bear this out.

But don't you have to take into account the fact that this is a female child and not a male child? If they said "a child" it would be 50%. However, since they specify a sex, don't you take that into account?

 

[Genemo]

The probability is built on you already have a daughter. If it mean to ask the way you understand, the question should be like "what is the probability that the child of these two parents will be a female carrier of a recessive allele? "

 

[Lexicon0]

In the case of an autosomal disorder it would be safe to assume that the gender of the child is completely irrelevant. In this case, if you completely disregard gender, a punnett square will yield a 50% chance of a daughter (or a child of any gender for that matter) being a carrier.

 

[dsoz]

I came across this practice question in my studies and I feel like it is incorrect.

39) Assume the autosomal condition X is caused by having two recessive alleles. If a father is a carrier of condition X and the mother does not have condition X (nor is she a carrier), what is the probability that a daughter of these two parents will be a carrier of a recessive allele?

A)100%
B) 75%
C) 50%
D) 25%

I said D, because there's a fifty percent chance you will have a female child, and a fifty percent chance one of the offspring is a carrier. Thus, (1/2)(1/2) = 1/4 = 25%.

Don't cut out the 50% for being female, because the question clearly states how many DAUGHTERS are affected... You don't need to worry about the 50% that are male!

dsoz

 

[coyotelime]

Father = carrier of condition X, so doesn't have the condition. Since it's autosomal recessive, he only has one copy of the condition X recessive allele.

Mother = none

Daughter = XX, one X from mother, one from father, which is probably assumed for this problem

One X will be good no matter what, because it's from the mom

Other X will be from dad, but dad has a good allele and a "bad" allele.

50%!

What book was this from?

Edit: I see what you were saying! They're saying IF the couple had a daughter, what would be the chances of her having an allele for condition X?

So 100% they have a daughter, multiply the answer by 1 if you want =9

 

[docelh]

I came across this practice question in my studies and I feel like it is incorrect.
39) Assume the autosomal condition X is caused by having two recessive alleles. If a father is a carrier of condition X and the mother does not have condition X (nor is she a carrier), what is the probability that a daughter of these two parents will be a carrier of a recessive allele?

A)100%
B) 75%
C) 50%
D) 25%

I said D, because there's a fifty percent chance you will have a female child, and a fifty percent chance one of the offspring is a carrier. Thus, (1/2)(1/2) = 1/4 = 25%.

However, the explanation said C was correct because you did not need to take the gender of the offspring into account because this was not a sex-linked trait. WTF? Has anyone heard of treating a problem like this using this logic before? I've always taken the gender into account regardless of whether or not it was sex-linked, given a specific gender was asked for in the question stem...!

I think you guys got "fooled" by some irrelevant details highlighted in red (daughter, use of X). The key phrase in here is autosomal, which refers the other 22 chromosomes, EXCLUDING the sex chromosome.

Because it's autosomal, the probability is the same for a son or a daughter.

 

[whiteshadodw]

I think you guys got "fooled" by some irrelevant details highlighted in red (daughter, use of X). The key phrase in here is autosomal, which refers the other 22 chromosomes, EXCLUDING the sex chromosome.

Because it's autosomal, the probability is the same for a son or a daughter.

not only that, but if the question asked what is the possibility of an offspring being female AND being a carrier then you would do

P(female)*P(carrier)=25%

 

[Doubtnichts]

It's asking the probability that a daughter will be a carrier...not the probability that they will have a daughter and she will be a carrier...it's a given that they already have a daughter the question is whether she is a carrier or not.

 

[SeminoleVesicle]

In the case of an autosomal disorder it would be safe to assume that the gender of the child is completely irrelevant. In this case, if you completely disregard gender, a punnett square will yield a 50% chance of a daughter (or a child of any gender for that matter) being a carrier.

👍

 

[docelh]

It's asking the probability that a daughter will be a carrier...not the probability that they will have a daughter and she will be a carrier...it's a given that they already have a daughter the question is whether she is a carrier or not.

👍

That's how I read it, although I can see how others could over-analyze the question. The give-away for me is that the question is trying to cross you up between sex-linked vs. autosomal, so it's highly unlikely they'll give you the confusing answer.

 

[raltima07]

Autosomal is the key word