Trisomy

A trisomy is a form of aneuploidy with the presence of three copies, instead of the normal two, of a particular chromosome.^[1] The presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.

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Causes

Full

Full trisomy of an individual occurs due to non-disjunction when the cells are dividing (meiosis I or II) to form egg and sperm cells (gametogenesis). This can result in an extra or missing chromosome (either 24 or 22 chromosomes instead of the typical 23) in a sperm or egg cell. After fertilization, the resulting fetus has 47 chromosomes instead of the typical 46.

Partial/mosaic

A partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes, or if one of the chromosomes has two copies of part of its chromosome.

A mosaic trisomy is a condition where extra chromosomal material exists in only some of the organism's cells.

Human trisomy

Trisomies can occur with any chromosome, but often result in miscarriage. For example, Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester. The most common types of trisomy that survive to birth in humans are:

Autosomal trisomies

• Trisomy 21 (Down syndrome)
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)
• Trisomy 12 (A prognostic indicator of Chronic Lymphocytic Leukemia)
• Trisomy 9
• Trisomy 8 (Warkany syndrome 2)

The most common forms of autosomal trisomy are trisomy of chromosome 21 which results in Down syndrome and trisomy of chromosome 18 which results in Edwards syndrome. In rare cases, a fetus with trisomy of chromosome 13 can survive, giving rise to Patau syndrome. Autosomal trisomy can be associated with birth defects, mental retardation and shortened life expectancy.

Sex-chromosome trisomies

Trisomy of sex chromosomes can also occur:

• XXX (Triple X syndrome)
• XXY (Klinefelter syndrome)
• XYY (XYY syndrome)

References