X-Linked Hyperphosphatemic Rickets

Rett's Syndrome

Incontinentia Pigmenti Type 2

Aicardi Syndrome

Huntington's Disease

(Inheritance)

Neurofibromatosis (Types 1 and 2)

(Inheritance)

Marfan Syndrome

(Inheritance)

Cystic Fibrosis

(Inheritance)

Autosomal Recessive

1:2:1

Sickle Cell Disease

(Inheritance)

Autosomal Recessive

1:2:1

Tay-Sachs Disease

(Inheritance)

Autosomal Recessive

1:2:1

Niemann-Pick Disease

(Inheritance)

Autosomal Recessive

1:2:1

Spinal Muscular Atrophy

(Inheritance)

Autosomal Recessive

1:2:1

Hemophilia A

(Inheritance)

Duchenne Muscular Dystrophy

(Inheritance)

Lesch-Nyhan Syndrome

(Inheritance)

Becker Muscular Dystrophy

(Inheritance)

G6PD Deficiency

(Inheritance)

• Never passed from father to son (True of X-Linked Dominant as well)
• Males much more likely to be affected
• If affected males are sterile, only males will be affected
• All affected males in a family are related through their mothers
• Trait or disease is typically passed from an affected grandfather (through his carrier daughters) to 1/2 of his grandsons 

• Never passed from father to son (True of X-Linked Recessive as well)
• All daughters of an affected male are affected.
• All sons of an affected male and normal female are normal
• 1/2 of sons and 1/2 of daughters of affected female and normal male are affected
• Males are usually more severely affected. The trait may be lethal in males
• Females are more likely to be affected, even if the disease is not lethal in males

X-Linked Dominant

The key for determining if a dominant trait is X-linked or autosomal is to look at the offspring of the mating of an affected male and a normal female. If the affected male has an affected son, then the disease is not X-linked. All of his daughters must also be affected if the disease is X-linked.

(I.E- ALL affected daughters, NO affected sons)

X-Linked Dominant (Lethal in males)

This is not uncommon in X-linked dominant diseases. There are no affected males to test for X-linked dominant inheritance to see if they produce all affected daughters and no affected sons. There are no affected males, only affected females, in the population. Living females outnumber living males two to one when the mother is affected. The ratio in the offspring of affected females is: 1 affected female: 1 normal female: 1 normal male.

Note the multiple spontaneous abortions

• Every affected individual has an affected biological parent. There is NO  no "skipping a generation"
• M:F ratio is 1:1
• Recurrence risk of each child of an affected parent is 1/2 (50%)
• Normal siblings of affected individuals do not transmit the trait to their offspring
• The defective product of the gene is usually a structural protein, not an enzyme

• M:F ratio is 1:1
• Recurrence risk to next child of affected individual is 1/4 (25%)
• Trait is characteristically found in siblings, but not parents OR offspring of affected individual
• Parents of affected children MAY be related. The rarer the trait, the more likely consanguinity
• Trait may appear as an isolated (sporadic) event in small sibships