name a disorder  karyotype analysis, pick and explain a disorder associated with a variation in chromosome number type of variation in chromosome number.  causes of variation.  phenotypic symptoms of disorder. name a disorder.

The example of disorder that can be taken in the current scenario which results from variation in chromosome number is Down's syndrome or 21st Trisomy.

Down's syndrome occurs due to an error that occurs during separation of the sister chromatids from chromosomes during meiosis or reduction division at the 21st chromosomal position. Since the separation does not take place, one of the two cells possesses more than the actual desired karyotype which is present in the meiotic cells (which usually possesses either 21+XX or 21+XY). Due to the nondisjunction of the 21st chromosome, one of the cells contains (22+XX or 22+XY) +1 chromosome which results in 47 chromosomes during fusion of sperm and ova. That is why this chromosomal aberration is known as 21st Trisomy, as 21st chromosome does not separate into two sister chromatids and leads to formation of 3 copies of the 21st chromosome (or trisomy) ar the 21st position of the newly formed zygote.