X-linked inheritance.

  1. Fathers usually pass a recessive allele to his daughter as a carrier.

  2. The carrier has a 50% chance of passing the disease to her sons.

  3. If the carrier marries an affected male, 50% of the offspring will have the disorder.

These recessive disorders are more common in males than in females, since females have 2 X chromosomes and need to be homozygous to express the trait.