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The extra chromosome content can arise through several different ways. The most common cause (about 92–95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21. [92] [97] In 1–2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic Down syndrome.
Jérôme Jean Louis Marie Lejeune (French pronunciation: [ʒeʁom ʒɑ̃ lwi maʁi ləʒœn]; 13 June 1926 – 3 April 1994) was a French pediatrician and geneticist, best known for his work on the link of diseases to chromosome abnormalities, most especially the link between Down Syndrome and trisomy-21 and cri du chat syndrome, amongst several others, and for his subsequent strong opposition ...
Chromosome 21 is both the smallest human autosome and chromosome, [4] with 46.7 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 (trisomy 21) have Down syndrome.
Trisomy 21, one of the three types of Down syndrome, indicates that an individual has a duplicate of chromosome 21. The extra chromosome changes how one’s brain and body develop, and can lead to ...
This chromosome was designated chromosome 21 in 1960. [4] Turpin discovered, in 1959, the first structural chromosomal abnormality translocation, the second of the two main types of chromosomal abnormalities. In 1947, Turpin founded the French Genetics Society, of which he became president in 1954.
A baby with Down syndrome has a full or partial copy of chromosome 21. ... The most common type of Down syndrome is trisomy 21, which accounts for 95% of all cases.
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. [4]
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