Down's syndrome is caused by an extra copy of which chromosome?

1. Chromosomal disorders arise from an abnormal number of chromosomes. 2. Failure of chromatids to segregate during cell division causes a gain or loss, called aneuploidy. 3. Down's syndrome results in the gain of an extra copy of chromosome 21. 4. That gain of one whole chromosome is why it is described as a trisomy. _Source: syllabus Biology Ch4 'Principles of Inheritance and Variation'_