An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X and Y). Autosomes are numbered roughly in relation to their sizes. That is, Chromosome 1 has approximately 2,800 genes, while chromosome 22 has approximately 750 genes.
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An autosome is one of the 22 numbered pairs of chromosomes that most of us carry in almost all of the cells of our body. We actually have a total of 23 pairs of chromosomes in these cells, for a total of 46 chromosomes, but two of those are referred to by letter rather than by number and are called sex chromosomes rather than autosomes, since they--that is the X and Y chromosome--help determine what sex, or gender, we are. The 22 pairs of autosomes are referred to by number basically in inverse correlation with their size. That is, Chromosome 1, with the smallest number, is actually the largest chromosome. It has almost 3,000 genes on it. And we go down to the smallest chromosomes, the ones with the largest numbers. You think that would be Chromosome 22, since we have Chromosomes 1 through 22, which only has about 750 genes, but in fact Chromosome number 22 is not the smallest of the autosomes. We thought it was when it was first described, so that's how it got named 22. It turns out that Chromosome 21 is actually a little bit smaller than Chromosome 22.
Alan E. Guttmacher, M.D.
Former Acting Director, National Human Genome Research Institute
Dr. Alan Guttmacher is the former acting director of the National Human Genome Research Institute, helping oversee the institute's efforts in advancing genome research, integrating the benefits of genome research into healthcare, and exploring the ethical, legal and social implications of human genomics. Dr. Guttmacher received his M.D. from Harvard Medical School in 1981. From 1982 to 1985, he completed an internship and residency in pediatrics at Children's Hospital Boston. In 1985, he earned a two-year National Research Service Award from the US Public Health Service as a fellow in medical genetics at Children's Hospital Boston and Harvard Medical School.