Sex linked recessive calculation

[BeatMCAT]

So, I was attempting this one problem today and got it wrong. question was that the woman is X Xc while man is XY. what is the probability that all three of their daughters have the disease?

The answer was 1/2 * 1/2 *1/2 = 1/8

I thought it would be 1/4*1/4*1/4 = 1/64 because there is 1/2 probabaility that it will be a son and 1/2 of those sons will have the disease. Am i getting something wrong? Thanks!!!

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[PingPongPro]

The way the question is worded, they presuppose that all their children are only daughters. So you only account for the girls.

 

[BeatMCAT]

The way the question is worded, they presuppose that all their children are only daughters. So you only account for the girls.

so how would it be worded if they dont have that assumption made already?

 

[PingPongPro]

What is the probability that all three of their children are female AND have the disease?

 

[BeatMCAT]

What is the probability that all three of their children are female AND have the disease?

ohh i see. thanks. appreciate it!

 

[Xivilus1231]

whoa, looking at this post is confusing me,

if the disease is sex-linked recessive, the children could either be...

Xc X - female carriers

or

Xc Y - male affected

If the disease is recessive, the daughters will be heterozygous for the disease but would not HAVE the disease?

 

[Trisphorin]

^^^agreed

if its sex linked recessive, how could the carrier have the disease?

 

[user003]

If the OP posts the question exactly how it was worded, I'm sure it would clear up alot of confusion for everyone

 

[BeatMCAT]

my bad. i am an idiot. i meant to say probability of sons having the disease. NOT daughter. sorry for the confusion.

 

[plzNOCarribbean]

whoa, looking at this post is confusing me,

if the disease is sex-linked recessive, the children could either be...

Xc X - female carriers

or

Xc Y - male affected

If the disease is recessive, the daughters will be heterozygous for the disease but would not HAVE the disease?

yeah I was looking at it and thinking, "geez I better review my genetics again bc something doesn't make sense".

if ALL 3 sons would have the disease, then the probability would be:

1/2 x 1/2 x 1/2 = 1/8

the sons get the Y chromosome from dad, and they have a 1/2 chance of getting the diseased allele from the mom.