How many pairs of chromosomes do humans have?

Answer

Humans have 46 chromosomes in 23 pairs, one set from each parent.

Context

Most human cells are diploid, meaning they carry two copies of the genome, one inherited from each parent. Those copies are packaged into 46 chromosomes arranged as 23 pairs: 22 pairs of autosomes numbered roughly by size, plus one pair of sex chromosomes, usually XX in females and XY in males. Eggs and sperm are haploid, with 23 chromosomes each, and fertilization restores the total of 46.

The number was settled relatively late. For decades after chromosomes were first described, many researchers believed that humans had 48, a count that held until 1956, when Joe Hin Tjio and Albert Levan used improved preparation methods on cultured cells and reported 46. Chromosome number has no link to complexity: chimpanzees and other great apes have 48 and a dog has 78, while some ferns have more than a thousand.

Changes in number can have consequences. Down syndrome, for example, usually results from an extra copy of chromosome 21, and Turner syndrome typically involves a single X chromosome. Not every cell follows the 46 rule either: mature red blood cells in humans lack a nucleus and therefore carry no chromosomes, and liver cells can be polyploid. Chromosome counts are therefore best read as a description of a species' packaging of DNA, not of its size.