Question

There are two main types of chromosomal disorders, those that take place on the 21st pair of chromosomes and those that ______________.

Answer 1

Down syndrome and Alzheimer's disease.

Down syndrome is a chromosomal disorder which is caused by the presence of extra copy of chromosome  21. Therefore also called as Trisomy 21.

Alzheimer's disease is a most common form of dementia(reduction in brain functioning). It is caused by mutation in single gene APP (Amyloid Precursor Protein) present on chromosomer 21.