Summary
Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is both the smallest human autosome and chromosome, with 45 million base pairs (the building material of DNA) representing about 1.5 percent of the total DNA in cells. Most people have two copies of chromosome 21, while those with three copies of chromosome 21 have Down syndrome, also called "trisomy 21". Researchers working on the Human Genome Project announced in May 2000 that they had determined the sequence of base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced, after chromosome 22. The following are some of the gene count estimates of human chromosome 21. Because researchers use different approaches to genome annotation, their predictions of the number of genes on each chromosome varies (for technical details, see gene prediction). Among various projects, the collaborative consensus coding sequence project (CCDS) takes an extremely conservative strategy. Thus CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes. Genes on human chromosome 21 The following is a partial list of genes on human chromosome 21. For complete list, see the link in the infobox on the right. The following diseases and disorders are some of those related to genes on chromosome 21: Alzheimer's disease Amyotrophic lateral sclerosis Atrial fibrillation, familial Autoimmune polyendocrine syndrome Basal ganglia calcification Bethlem myopathy Closed angle glaucoma Cataract Down syndrome Erondu–Cymet syndrome Holocarboxylase synthetase deficiency Homocystinuria Hyperhomocysteinemia Hypotrichosis Immunodeficiency Inflammatory bowel disease Intellectual developmental disorder Jervell and Lange-Nielsen syndrome Keppen–Lubinsky syndrome Leukocyte adhesion deficiency-1 Majewski osteodysplastic primordial dwarfism type II (MOPD II, or MOPD2) Non-small cell lung carcinoma Nonsyndromic deafness Peripheral neuropathy Primary ciliary dyskinesia Romano–Ward syndrome Ullrich congenital muscular dystrophy Unverricht–Lundborg disease, a form of progressive myoclonus epilepsy The following conditions are caused by changes in the structure or number of copies of chromosome 21: Cancers: Rearrangements (translocations) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer.
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