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Pedigree

In the above pedigree, the black square indicates the male affected with Hemophilia which is a X-linked recessive trait. What is the probability that the proposed child, which is a male (indicated as ?), will carry the disease?

I started off by trying to figure out the probability that the sister has an X chromosome carrying the disease, which I got as 10/16 (Since the mother must be XcXc or XcX and the father either XY or XcY; possible genotypes are XcX, XcX, XcXc, XcXc, XcX, XX, XcXc, XXc)

I then repeated the procedure for each generation and got the final answer as 9/16. However, the correct answer is 1/8. How do you obtain this? Are my assumptions flawed?

Source: https://biolympiads.com/wp-content/uploads/2016/08/inbo2009-Q.pdf

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Assume the disease is rare. The parents of the affected male are thus probably X/Y and Xc/X. Their daughter (the sister of the affected male) has a probability of 1/2 of being a carrier (Xc/X). Her daughter also has a 1/2 probability of being a carrier. Her son has a 1/2 probability receiving the mutant X chromosome. Therefore, the probability that the son is affected is 1/2 * 1/2 * 1/2 = 1/8.

I can’t right now, but I’ll try and make it clearer with a figure tomorrow.

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