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Chromosome 21 from Human Genome Program Normal 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 ...
Down syndrome or Down's syndrome, [12] also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [3] It is usually associated with developmental delays, mild to moderate intellectual disability , and characteristic physical features.
Research of Down syndrome–related genes is based on studying the genes located on chromosome 21. In general, this leads to an overexpression of the genes. [1] [2] Understanding the genes involved may help to target medical treatment to individuals with Down syndrome. It is estimated that chromosome 21 contains 200 to 250 genes. [3]
Mosaic Down syndrome: Only some cells in the person have an extra copy of chromosome 21. This genetic cause is considered rare. This genetic cause is considered rare.
Trisomy 21 – Down syndrome, an example of a polysomy at chromosome 21. Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an organism has at least one more chromosome than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. [1]
Turner syndrome results from a single X chromosome (45,X or 45,X0). Klinefelter syndrome, the most common male chromosomal disease, otherwise known as 47,XXY, is caused by an extra X chromosome. Edwards syndrome is caused by trisomy (three copies) of chromosome 18. Down syndrome, a common chromosomal disease, is caused by trisomy of chromosome 21.
“In the most common type of Down syndrome, there are 47 chromosomes, with the extra one being chromosome 21 — so the patient has three No. 21 chromosomes in all the cells of their body.
Down syndrome: 1:600 Chromosome 21 Klinefelter syndrome: 1:500–1000 males Additional X chromosome Turner syndrome: 1:2000 females Loss of X chromosome Sickle cell anemia: 1 in 50 births in parts of Africa; rarer elsewhere β-globin (on chromosome 11) Bloom syndrome: 1:48000 Ashkenazi Jews BLM Cancers Breast/Ovarian cancer (susceptibility)