Red-green colour blindness is caused by a recessive allele on the X chromosome. A carrier woman marries a man with normal vision. What proportion of their sons is expected to be colour blind?

The mother is X$^C$X$^c$ and the father X$^C$Y. Sons receive their only X chromosome from the mother, so half inherit X$^c$ and are colour blind. All daughters receive the father's normal X$^C$ and have normal vision, though half are carriers.