An autosome is a chromosome that belongs to a group of homologous chromosomes; in diploid organisms, homologs form pairs. The DNA contained in homologous chromosomes is essentially the same, with minor variations arising from mutations. These minor variations roughly correspond to the alleles of genes. Humans have 22 pairs of autosomes in their 23 total pairs of chromosomes (the final pair are the X and Y allosomes, or sex chromosomes).

During meiosis, autosomes can exchange intervals of DNA, a phenomenon called recombination that serves an important evolutionary purpose in that it increases the possible diversity of gametes produced. In contrast, allosomes do not recombine.