Chromosome 21 from Human Genome ProgramNormal human Karyotype
Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on chromosome 21, either in whole (trisomy 21) or part (such as due to translocations). The effects of the extra copy varies greatly from individual to individual, depending on the extent of the extra copy, genetic background, environmental factors, and random chance. Down syndrome can occur in all human populations, and analogous effects have been found in other species, such as chimpanzees and mice. In 2005, researchers have been able to create transgenic mice with most of human chromosome 21 (in addition to their normal chromosomes).[1]
A typical human karyotype is shown here. Every chromosome has two copies. In the bottom right, there are chromosomal differences between males (XY) and females (XX), which do not concern us. A typical human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement for females and 46 chromosomes with an XY arrangement for males.[2] For this article, we will use females for the karyotype designation (46,XX).
^BBC News (22 September 2005). "Down's syndrome recreated in mice". Retrieved 2006-06-14.
^For a description of human karyotype see Mitelman, F. (1995). ISCN 1995 : an international system for human cytogenetic nomenclature (1995) : recommendations of the International Standing Committee on Human Cytogenetic Nomenclature, Memphis, Tennessee, USA, October 9-13, 1994.
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