Which of the following is not a Mendelian disorder?
Alzheimer's disease
Huntington's disease
Cystic fibrosis
Sickle-cell anaemia
Alzheimer's disease is not considered a Mendelian disorder as it is not caused by a single gene mutation but is rather influenced by multiple genes and environmental factors. In contrast, the other options listed are Mendelian disorders caused by mutations in a single gene. Hence option A is the correct answer. Hence, option 1 is the correct answer.