Inheritance of a genetic disorder

Since the probability for the disease to pass to male and female offsprings is different, the disease is X-linked.
If we symbolize by Xa the faulty chromosome and X the normal chromosome, then a male person with the disease has genotype XaY, while the normal male has XY.
A female person with the disease has genotype XaX (we assume that the person is heterozygous because the exercise mentions that "females with the disorder will pass it on to approximately half of their children."
A normal female person has genotype XX.

The punnett squares are at the attachment (red squares = children with the disease)

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