Which type of inheritance does Incontinentia pigmenti fall into?

• A. X-linked dominant
• B. Autosomal recessive
• C. Y-linked
• D. Autosomal dominant

Answer: (A)

Incontinentia pigmenti is associated with an X-linked dominant inheritance pattern. This means that the gene responsible for the condition is located on the X chromosome and that a single copy of the mutated gene can cause the disorder in females, who have two X chromosomes, as well as in males, who have one X and one Y chromosome. In affected females, the presence of one mutated X chromosome is sufficient to express the condition, while males with the mutation often have more severe manifestations or may not survive to term.

In X-linked dominant conditions, affected females can pass the mutation on to their children with a 50% chance for both sons and daughters to inherit the affected X chromosome. This characteristic pattern of inheritance is what makes Incontinentia pigmenti accurately categorized as X-linked dominant. Other inheritance types, such as autosomal recessive or autosomal dominant, do not apply here as they involve different modes of inheritance and typically involve genes located on autosomes rather than the sex chromosomes. Y-linked inheritance would be completely inappropriate, as it would only affect males and would not include females in the pedigree of the condition. Thus, understanding the X-linked dominant inheritance pattern helps clarify how Incontinentia pigmenti is passed through generations.