X-linked recessive disorders

red/green colour blindness,
Duchenne and Becker muscular dystrophies,
Fragile X Syndrome,
G6PD deficiency,
haemophillias A
Hunter's Syndrome.

The abnormal gene is carried on he X chromosome,
and in the carrier female, the normal allele on her other
X chromosome protects her from the disease.
Since the male does not have this protection,
he manifests the disease.

In X-linked inheritance therefore:

• Males are all affected.
• Females only occasionally show mild sign of disease.
• Each son of a female carrier has a 1:2 chance of being affected.
• Each daughter of a female carrier has a 1:2 risk of being a carrier.
• Daughters of affected males will all be carriers,
• but sons of affected males will not be affected since the Y chromosome is derived from father.