Autosomes are chromosomes that are not sex chromosomes. They exist in pairs in somatic cells and carry genetic information that governs numerous biological processes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Autosomes are numbered roughly in relation to their sizes, with chromosome 1 being the largest and chromosome 22 being the smallest. The largest autosome, chromosome 1, has approximately 2,800 genes, while the smallest autosome, chromosome 22, has approximately 750 genes. Autosomes control the inheritance of all an organisms characteristics except the sex-linked ones, which are controlled by the sex chromosomes. Numerical abnormalities in the autosomes are generally thought to result from meiotic nondisjunction, which can occur during either maternal or paternal gamete formation. Possessing a single copy of an autosome (known as a monosomy) is nearly always incompatible with life, while having three copies of an autosome (known as a trisomy) is far more compatible with life. Down syndrome is a common example of trisomy, which is caused by possessing three copies of chromosome 21 instead of the usual two.