Fragile X Syndrome is due to mutation on which chromosome and tends to affect which gender more?

• A. Mutation on the X chromosome; more common in boys
• B. Mutation on the Y chromosome; more common in boys
• C. Mutation on chromosome 21; affects both genders equally
• D. Mutation on the X chromosome; affects girls more

Answer: (A)

Fragile X syndrome results from a mutation on the X chromosome. It is inherited as an X-linked pattern, so males are more likely to be and be more severely affected because they have only one X chromosome; a single mutated copy leads to fuller expression. Females have two X chromosomes, so the normal X can compensate in many cells, and due to random X-inactivation the condition is usually milder or variable in them. Therefore, the mutation is on the X chromosome and tends to affect boys more. It is not on the Y chromosome, and not linked to chromosome 21.