UWorld genetics question

[CrimsonMirage]

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UWorld stated that:
probability that a female --- with a brother who has an X-linked disorder -- will give birth to an affected child is 1/8. (her partner is unaffected).

I still don't get how they came up with 1/8. For a male with an X-linked disorder, what is the risk that a brother or a sister is affected? What are the chances of the sister being a carrier?

Aaah sorry if that was confusing. Any help appreciated!! Thanks

 

[MrBeauregard]

(50% chance, or 0.5, that she is a carrier) x (50% chance, or 0.5, that she passes the mutant allele) x (50% chance, or 0.5, that the child is a boy and can be affected) = 0.125, or 1/8