You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is Down's syndrome is most commonly caused by ? and explain in detail?
Ans. is. 'a' i.e., Maternal nondisjunction o The most common cause of trisomy 21 (Down's syndrome) is maternal nondisjunction in meiosis 1. In most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum and in 95% of the cases the extra chromosome is of maternal origin.