2. X-linked recessive inheritance
More than 1000 genes have been identified on the X chromosome, whereas only about 200 are believed to be present on the Y chromosome.

3. Rules of XR inheritance:
Males are more commonly and more severely affected than females.
Female carriers are generally unaffected, or if affected, they are affected more mildly than males.
Female carriers have a 25% risk for having an affected son, a 25% risk for a carrier daughter, and a 50% chance of having a child that does not inherit the mutated X-linked gene.
Trait is transmitted from affected males to all of their daughters; it is never transmitted father to son because they pass their X chromosome to all of their daughters and their Y chromosome to all of their sons.
Male-to-male transmission excludes X-linkage but is seen with autosomal dominant and Y-linked inheritance.
Family history may be negative - new mutations and gonadal mosaicism.
Identifying female carriers is important to be able to provide genetic counselling.

5. Pedigree of Autosomal Recessive Inheritance

6. E.g. of some XR inheritance
System
Disease
Musculoskeletal
Duchenne muscular dystrophy
Blood
Hemophilia
Glucose 6 phosphate dehydrogenase
Eye
Color blindness
CNS
Fragile X syndrome
Metabolic
Diabetes inspidus
Hunters (MPS)
Immune
Agammaglobulinemia
Wiskott aldrich syndrome

8. X-linked dominant inheritance
Only few X linked dominant disorders have been described.
Both male and female affected, but female have less severe symptoms due to X- chromosome inactivation, e.g. hypophsphatemic rickets.
Many XD disorders are lethal in male, e.g. incontinentia pigmenti.
An affected man will have only affected daughters and unaffected sons, and half of the offspring of an affected woman will be affected .

12. Good Luck