Rett syndrome is caused by mutations in the MECP2 gene and follows an X-linked dominant pattern of inheritance. Which of the following statements is therefore true?

• A. It takes two defective alleles for the condition to be phenotypically expressed in females.
• B. It only affects females.
• C. The trait will skip in one generation.
• D. Once one allele is defective, the condition is expressed phenotypically.

Answer: (D)