Patterns of Inheritance (Part 5)

Question 1

X – Linked Recessive Disorders (Characteristics)

Answer 1

• Males require only one copy of the mutation (hemizygous) to express the disease. More common in males than in females
• Skipped generations common
• Affected father transmits the mutation to all his daughters who are NOT affected and are carriers. The daughters transmit the mutation to their sons
• Male to male transmission is not seen

Question 2

X-linked recessive - Mothers of affected sons are obligate carriers of the mutant gene

Answer 2

X-linked recessive - Mothers of affected sons are obligate carriers of the mutant gene

Question 3

X- linked recessive - Daughters of affected males are obligate carriers

Answer 3

X- linked recessive - Daughters of affected males are obligate carriers

Question 4

All daughters of a man affected with an X-linked disorder are heterozygous carriers of the mutation.

Answer 4

All daughters of a man affected with an X-linked disorder are heterozygous carriers of the mutation.

Question 5

Manifesting Heterozygotes

Answer 5

Manifestations in a female are most commonly due to skewed X-inactivation (asymmetric X inactivation)
• If this asymmetric X-chromosome inactivation occurs in the liver, the carrier may have a factor VIII activity level below normal and thus experience bleeding problems.

• due to skewed X inactivation in these females (the number of cells that contain the active mutant X are large compared to the cells that contain the active normal X)

Question 6

X – Linked Dominant Disorders (Characteristics)

Answer 6

• Skipping of generations not common
• Preponderance of females compared to males
• No male to male transmission
• Affected male transmits the disease to all his daughters, but none of his sons would be affected

Question 7

X – Linked Dominant Disorders

Answer 7

• Vitamin D resistant Rickets (hypophosphatemic rickets)

* Incontinentia pigmenti

Question 8

Examples of Female Mosaics

Answer 8

• Incontinentia pigmenti causes patchy, darker pigmentation where the normal X has been inactivated (mutant X is active)
• Areas of normal pigmentation indicate the areas where the normal X is active

Question 9

X-linked disorders that is lethal in males

Answer 9

Rett syndrome

Question 10

Y-linked Inheritance

Answer 10

• Only males are affected.
• Genes on Y are primarily genes involved in spermatogenesis and, therefore, mutations usually cause sterility and are not passed on.