biology question!!

[yingao]

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if an X-linked recessive disorder affects 1/1000000 females in a population, what is the expected frequency of affected males in the population?

a. 0
b. 1/10
c.1/100
d. 1/1000

ans: d.

 

[user003]

I'm assuming you need an explanation:

Females have XX, males have XY. Since the disease is X-linked recessive, females require two X chromosomes with the same mutation to have the disease, but males require only one X chromosome with the mutation.

Disease frequency in females is 1/1,000,000. In that case, the frequency at which the mutation is present in each X chromosome is 1/1,000 (1/1,000 x 1/1,000 = 1,000,000)

Males have XY, then their disease frequency has to be the same as that of a single X chromosome: 1/1,000

 

[MShopes]

I'm assuming you need an explanation:

Females have XX, males have XY. Since the disease is X-linked recessive, females require two X chromosomes with the same mutation to have the disease, but males require only one X chromosome with the mutation.

Disease frequency in females is 1/1,000,000. In that case, the frequency at which the mutation is present in each X chromosome is 1/1,000 (1/1,000 x 1/1,000 = 1,000,000)

Males have XY, then their disease frequency has to be the same as that of a single X chromosome: 1/1,000

Your explanation is great. However, by the word affected in the question, couldn't the female be only a carrier and thus one X chromosome needs to be mutated to be affected. The question is lacking a clearance a bit. The word affected could mean expressing the disease (two X chromosomes are affected for the female) or just a carrier (just one X chromosome is affected and thus the dominant is healthy overall). If however they meant the disease in the question, then I agree with you 100 %.

 

[harryking]

I was a teaching assistant for genetics and I think it is just the convention in genetics. Think about pedigree analysis, "affected" will always mean individuals who express the phenotype and you wouldn't call a carrier individual "affected" in a pedigree analysis.

 

[MShopes]

I was a teaching assistant for genetics and I think it is just the convention in genetics. Think about pedigree analysis, "affected" will always mean individuals who express the phenotype and you wouldn't call a carrier individual "affected" in a pedigree analysis.

Okay that makes sense since a carrier don't really shows that they have the disease at all in their phenotype. So, from a physical appearance, they aren't affected. I just didn't like to risk it by taking the fact that affected refers to only disease and not a carrier. Now, if they don't mention the word disease, I will take your word and assume that affected don't refer to a carrier.

 

[user003]

Your explanation is great. However, by the word affected in the question, couldn't the female be only a carrier and thus one X chromosome needs to be mutated to be affected. The question is lacking a clearance a bit. The word affected could mean expressing the disease (two X chromosomes are affected for the female) or just a carrier (just one X chromosome is affected and thus the dominant is healthy overall). If however they meant the disease in the question, then I agree with you 100 %.

I would agree with you, but the question states that the disease is X linked recessive. A female with only one mutation would be a carrier, thus she would not be affected. Her male offsprings have a 50% chance inheriting the mutated X chromosome and the disease.

I hope that clears things up!

 

[MShopes]

I would agree with you, but the question states that the disease is X linked recessive. A female with only one mutation would be a carrier, thus she would not be affected. Her male offsprings have a 50% chance inheriting the mutated X chromosome and the disease.

I hope that clears things up!

That's the point. I didn't know that a carrier wouldn't be called affected and thus assumed this assumption. I thought that as long one of the alleles is mutated and carry one X chromosome with the recessive trait of that disease, then she would be called affected. But now I understand that affected usually refers to the phenotype expressed to us (which would mean the two X chromosomes of the female must be affected and one X chromosome of the male only would be affected since the disease is expressed on that one X chromosome of the male any ways no matter if it is recessive or dominant trait).

Thanks for clearing it up!

 

[tn4596]

jsut curious, so lets say the disease is X-link dominant, if the female affected is 1/x, what is the ratio of male affected...
I was thinking if female have XX, they have double the chances of being affected (can get it both from mom and dad), so male ratio (can only get it from mom) would be 1/2x...
or should I solve it using the frequency of the allele, which probaly impossible to solve since this is a dominant disease...
Or...is this problem can not be solved using these information...

 

[MShopes]

jsut curious, so lets say the disease is X-link dominant, if the female affected is 1/x, what is the ratio of male affected...
I was thinking if female have XX, they have double the chances of being affected (can get it both from mom and dad), so male ratio (can only get it from mom) would be 1/2x...
or should I solve it using the frequency of the allele, which probaly impossible to solve since this is a dominant disease...
Or...is this problem can not be solved using these information...

I think they are the same ratio (1/x). Because the disease is X-linked dominant trait, it would only require to be on just one X for the female to be expressed and affected and it would require the same one X of the male (any ways its always just one X for the male) to be expressed. The same principle goes for this problem. Correct me if I'm wrong any one.

 

[MShopes]

jsut curious, so lets say the disease is X-link dominant, if the female affected is 1/x, what is the ratio of male affected...
I was thinking if female have XX, they have double the chances of being affected (can get it both from mom and dad), so male ratio (can only get it from mom) would be 1/2x...
or should I solve it using the frequency of the allele, which probaly impossible to solve since this is a dominant disease...
Or...is this problem can not be solved using these information...

Just for your info, if female has XX, the female has actually half the chance of being affected because both XX have to be mutated first to be affected which is more work than changing one X of the male. So the male would be affected easier since only one X for him need to be mutated. Hope you got my point.

 

[tn4596]

how can this be related to mutation, I was taught you get X from mom and, either X or Y from your dad, purely hereditary...

 

[MShopes]

how can this be related to mutation, I was taught you get X from mom and, either X or Y from your dad, purely hereditary...

Well I meant to talk about the mom and dad in the first place. They do get the disease through mutation most likely, although not always. Even if it isn't true, using the term mutation makes it easier for me to answer the question.

And the question didn't mention passing the genes to offspring. It was talking about a female vs male in an environment and what is the ratio of affected individuals. Since the disease happens sometimes because of mutation, the term mutation applies here.

 

[Laureatebarrel]

Thanks for great question and answer!