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Trisomy is a genetics term meaning the possession of three copies of a particular chromosome in species which normally possess two copies of each chromosome. It is caused when either a sperm or egg fail to become completely haploid prior to fertilization. The risk of trisomy increases with increased maternal, and to a lesser extent paternal, age at the time of conception. The risk of having a child with trisomy rises gradually with maternal age until the early 30's, at which point it rises sharply every year thereafter. The combined rate of all forms of trisomy in humans is approximately 1 in 800 live births, but trisomy also results in many miscarriages. Trisomy 16 (extra copy of chromosome 16), for example, is reported to occur in 1% of all human pregnancies but results in miscarriage before 12 weeks of pregnancy.

In humans, trisomy typically causes characteristic anatomic and developmental abnormalities. Some examples include:

Down Syndrome, caused by trisomy of human chromosome 21, which produces a characteristic group of facial, cardiac, and gastrointestinal abnormalities as well as variable degrees of developmental delay.
Edwards Syndrome, caused by trisomy of chromosome 18, usually results in miscarriage, but also produces facial and cardiac abnormalities in infants who survive to term.
Patau Syndrome, caused by trisomy of chromosome 13, results in microcephaly and cleft palate and a number of cardiovascular defects. It is frequently fatal soon after birth.

There can also be trisomy of human sex chromosomes, which can also result in abnormalities of growth and development.

Klinefelter Syndrome or 47XXY is caused by the addition of an extra X chromosome to the normal male XY pattern. The main effect is sterility and subtle developmental delays.
XYY Syndrome, in which there is an extra Y chromosome in addition to the normal male XY pattern, is by contrast associated with little if any anatomic or developmental abnormality in affected men. Similarly, XXX Syndrome representing a third X chromosome in addition to the normal female XX pattern is rarely diagnosed because of lack of symptoms in affected women.

Trisomy should not be confused with triploid, in which there are three copies of all chromosomes.