|
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 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. Recently, researchers have been able to create transgenic mice with most of human chromosome 21 (in addition to their normal chromosomes). A normal 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 normal 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.〔For a description of human karyotype see 〕 For this section, we will use females for the karyotype designation (46,XX). ==Trisomy 21== Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event.〔There is a nice animation that shows nondisjunction at (【引用サイトリンク】 title=Meiotic nondisjunction animation )〕 A normal gamete (either egg or sperm) has one copy of each chromosome (23 total). When it is combined with a gamete from the other parent during conception, the child has 46 chromosomes. However, with nondisjunction, a gamete is produced with an extra copy of chromosome 21 (the gamete has 24 chromosomes). When combined with a normal gamete from the other parent, the child now has 47 chromosomes, with three copies of chromosome 21. The trisomy 21 karyotype figure shows the chromosomal arrangement, with the prominent extra chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndrome, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete. Mitotic nondisjunction after conception would lead to mosaicism, and is discussed later. Some cases have been reported of Down syndrome parents having trisomy 21 children.〔For an example of mosaic Down syndrome mother, see 〕 In these cases (all from mothers), the ovaries were trisomy 21, leading to a secondary nondisjunction during gametogenesis and a gamete with an extra chromosome 21. Such Down syndrome trisomies are indistinguishable from Down syndrome trisomies created through meiotic nondisjunction. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Genetics of Down syndrome」の詳細全文を読む スポンサード リンク
|