DEFINATION: genetic disorder in which a person has three copies of chromosome instead of two

TYPES: autosomal and sex trisomy
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)

Trisomy of sex chromosomes can also occur and include:
• XXX (Triple X syndrome)
• XXY (Klinefelter syndrome)

PATHOPHYSIOLOGY
Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Humans have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.
If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes. If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

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