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.
Chromosomes are only visible when a cell is undergoing division. Otherwise, the DNA remains uncoiled in the cell's nucleus.
Chromosome pairs[]
Humans have 23 chromosome pairs, including an "XX" pair for genetic females and an "XY" pair for genetic males. The X and Y chromosomes are named for their distinct shapes but near-identical lengths. Apart from the XX-XY pair, they are designated by their overall length, with "1" being the longest and "22" the shortest.
Most humans have all chromosomes in pairs. However, a few genetic conditions are different.
- An extra chromosome in a set, meaning there are 3 chromosomes instead of 2, is called trisomy (e.g. Down syndrome, which is trisomy 21)
- A single chromosome in a set, meaning there is 1 chromosome instead of 2, is called monosomy (e.g. Turner syndrome, which is an X chromosome with no X or Y counterpart)
People with these conditions may experience a wide variety of symptoms. Some may be harmless, like unique facial features. Others may be life-threatening, like heart defects. Intellectual disability and developmental delay are common in many different chromosomal conditions.
Cancer[]
Some cancer cells have irregularly shaped chromosomes, which often are of random length and do not necessarily appear in ordered pairs.