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Trisomy 21 (47,XY,+21) is caused by a meiotic nondisjunction event. A typical 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.
Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. The majority of cases result from nondisjunction during maternal meiosis I. [14] Trisomy occurs in at least 0.3% of newborns and in nearly 25% of spontaneous abortions .
Trisomy 21. Down syndrome (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males) is mostly caused by a failure of the 21st chromosome to separate during egg or sperm development, known as nondisjunction. As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes.
Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis. Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21.
Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. In nondisjunction, a pair of chromosomes fails to separate during cell division; thus, a gamete ( i.e., a sperm or egg cell) is produced with an extra copy of chromosome (for a total of 24 chromosomes).
The presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21. Trisomy 18 and Trisomy 13 , known as Edwards syndrome and Patau syndrome , respectively, are the two other autosomal trisomies recognized in live-born humans.
The most common types of autosomal trisomy that survive to birth in humans are: Trisomy 21 (Down syndrome) Trisomy 18 (Edwards syndrome) Trisomy 13 (Patau syndrome) Trisomy 9; Trisomy 8 (Warkany syndrome 2) Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to Patau syndrome ...
In the case where the sperm is the genetic cause of 48,XXXY syndrome, the sperm would have to contain two X chromosomes and one Y chromosome. This would be caused by two nondisjunction events in spermatogenesis, both meiosis I and meiosis II. The duplicated X chromosome in the sperm would have to fail to separate in both meiosis I and meiosis ...
For example, carriers of Robertsonian translocations involving chromosome 21 have a higher risk of having a child with Down syndrome. This is known as a 'translocation Downs'. This is due to a mis-segregation (nondisjunction) during gametogenesis. The mother has a higher (10%) risk of transmission than the father (1%).
Trisomy 21, an extra copy of the 21st chromosome, is responsible for causing Down syndrome, and the mouse chromosome 16 closely resembles human chromosome 21. In 1979, trisomy of the mouse chromosome 16 (Ts16) initially showed potential to be a model organism for human Down syndrome.