autosome, any of the numbered or nonsex chromosomes of an organism. Humans have 22 sets of autosomes; they are referred to numerically (e.g., chromosome 1, chromosome 2) according to a traditional sort order based on size, shape, and other properties. Autosomes differ from sex chromosomes, which make up the 23rd pair of chromosomes in all normal human cells and come in two forms, called X and Y. Autosomes control the inheritance of all an organism’s 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—that is, the unequal division of chromosomes between daughter cells—that can occur during either maternal or paternal gamete formation. Meiotic nondisjunction leads to eggs or sperm with additional or missing chromosomes. Down syndrome is probably the best-known and most commonly observed of the autosomal trisomies. Trisomy 13 and trisomy 18 are other numerical abnormalities seen in human populations, albeit at greatly reduced rates compared with Down syndrome. Structural abnormalities of the autosomes also occur, including translocations of large pieces of chromosomes as well as smaller deletions, insertions, or rearrangements. Even a small deletion or addition of autosomal material—too small to be seen by normal karyotyping methods—can produce serious malformations and intellectual disability.