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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.
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 ]
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.
The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2n+1 if one chromosome shows trisomy, 2n+1+1 if two show trisomy, etc. [2] "Full trisomy", also called "primary trisomy", [2] means that an entire extra chromosome has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome.
An extra or missing chromosome is a common cause of some genetic disorders. Some cancer cells also have abnormal numbers of chromosomes. [3] [4] About 68% of human solid tumors are aneuploid. [4] Aneuploidy originates during cell division when the chromosomes do not separate properly between the two cells (nondisjunction).
A World Down Syndrome Day celebration in Brazil. World Down Syndrome Day (WDSD) is marked each year on March 21, [1] beginning in 2007. The 21st day of March (the 3rd month of the year) was selected to signify the uniqueness of the triplication of the 21st chromosome which causes Down syndrome.
Note that chromosome 21 is present in 3 copies, while all other chromosomes show the normal diploid state with 2 copies. Most cases of trisomy of chromosome 21 are caused by a nondisjunction event during meiosis I (see text). Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. [2]
In unbalanced forms, Robertsonian translocations cause chromosomal deletions or addition and result in syndromes of multiple malformations, including trisomy 13 (Patau syndrome) and trisomy 21 (Down syndrome). The most frequent forms of Robertsonian translocations are between chromosomes 13 and 14, 14 and 21, and 14 and 15.