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Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the
21st chromosome, either in whole (
trisomy 21) or part (such as due to
translocations). The effects of the extra copy vary greatly among people, depending on the extent of the extra copy, genetic history, and pure chance. Down syndrome occurs in all human populations, and analogous effects have been found in other species such as chimpanzees
[5] and mice. Recently, researchers have created
transgenic mice with most of human chromosome 21 (in addition to the normal mouse chromosomes).
[6] The extra chromosomal material can come about in several distinct ways. A typical human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement typical of females and 46 chromosomes with an XY arrangement typical of males.
[7]
Trisomy 21
Trisomy 21 (47,XX,+21) is caused by a
meiotic nondisjunction event. With nondisjunction, a
gamete (
i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the
embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.[8
Down syndrome - Wikipedia, the free encyclopedia
