Chromosome

In genetics, a chromosome is a distinct single piece of coded DNA combined with binding proteins within a cell. Organisms of the same species have the same distinct chromosome pairs which all contain the same alleles. Males and females differ due to whether one of the chromosomes is of a distinct type, designated "X" and "Y" from their distinct shape but near identical length. They're only visible when a cell is undergoing division. Otherwise, the DNA remains uncoiled in the cell's nucleus.
Humans have 23 chromosome pairs, including an "XX" pair for females and an "XY" pair in males. Apart from the XX-XY pair, they are designated by their overall length with "1" being the longest and "22" the shortest.
Some diseases are caused by either an extra chromosome (e.g. Trisomy-21) or a missing chromosome (e.g. Turner syndrome).
Some cancer cells have irregularly shaped chromosomes, which often are of random length and do not necessarily appear in ordered pairs.
Chromosome at Wikipedia
Category:Genetics