複選題11. About Down syndrome, which of the following statements is NOT correct? (A) Down syndrome is usually the result of an extra chromosome 21, which means that each body cell has 47 chromosomes. (B) The frequency of Down syndrome increases with the mother's age. (C) This is mainly caused by nondisjunction during meiosis I, but not meiosis II. (D) Chromosomal mutation can be easily studied by analysis of Karyotypes. (E) Recent data suggest that by age 35, nearly 100% of people with Down syndrome develop Alzheimer syndrome.