Trisomy is a genetic disorder in which a child is born with all or part of an extra chromosome. Normally humans have two copies of every chromosome (except for males who have a pair of different chromosomes in the form of the sex chromosomes the X and the Y). In Trisomy, a child is born with third copy or part of a third copy of a chromosome. This happens during the development of sex cells (egg and sperm). When the sex cells are going through cellular division (which is one stage of sex cell development) the chromosomes do not separate like they should. This is called non-disjunction and the result is that an egg or sperm cell has an extra chromosome when it is formed. The egg or sperm cell then carries this extra chromosome to the fertilization event. Which will mean the developing child will have this extra chromosome or trisomy.
Trisomy is not tolerated well humans and in many cases the developing child is miscarried before being carried to full term. However, there are a few cases of Trisomy in which a baby can be carried to full term and born, but the child normally have some type of disorder due to the presence of the extra chromosome. These would include Trisomy 18, Trisomy 21 and Trisomy of the sex chromosomes.
In Trisomy 18 or Edwards Syndrome, the child has an extra full or partial copy of chromosome 18. The survival rate for Trisomy 18 is not good. A child born with Trisomy 18 can have a variety of life-threating defects including heart defects; kidney problems; developmental delays; problems with the digestive system (intestine outside the body when born, and esophagus not connecting to stomach); fluid on the brain; and umbilical or inguinal hernia. These defects are so severe that less than 10% of children born with Trisomy 18 will ever celebrate their 1st birthday. However, some children with Trisomy 18 can live well into their 20s and 30s but they usually need constant care and cannot live an independent life.
In Trisomy 21 or Down syndrome, the child has an extra copy of chromosome 21. The survival rate for Trisomy 21 is a lot better than for Trisomy 18. Many children with Trisomy 21 survive well into their 50s and 60s. A child born with Trisomy 21 can have a variety of developmental problems, which vary from mild to severe. These defects can include heart defects at birth, Eye problems, hip problems (leading to dislocation), Sleep apnea, and hypothyroidism. They may also have delayed mental and social development, which can lead to slow learning, short attention span, poor judgment, and impulsive behavior. They may also have distinct physical characteristics including shorter stature (due to arrested development), excess skin at the nape of the neck, flattened nose, small ears and mouth. Their head may be smaller than normal and have an abnormal shape. Many people with Trisomy 21 need constant care for their entire lifetimes but some can have close to normal lives living in group homes that have special assistance for people with Trisomy 21.
Other types of trisomy include trisomy of the sex chromosomes including XXY and XYY in males and XXX in females. These normally don’t have the same developmental effects as trisomy 18 and 21. Trisomy of the sex chromosomes can cause an individual to be infertile and may cause some external physical effects such as breast development in males with XXY. However, individuals with trisomy of the sex chromosomes often have close to normal intelligence and can lead a normal life without the constant care need by individuals with trisomy 18 and 21.
There are no treatments for trisomy in humans. Only the symptoms of the disorder can be treated. The diagnosis for these disorders is done by a test on the cells to see if the cell is carrying extra chromosomes. This can be done after birth or during an amniocentesis test during pregnancy if your doctor feels you are at risk for having a child with a trisomy disorder. The chances of a woman giving birth to a trisomy child increases with her age.
References
http://www.trisomy18.org/site/PageServer?pagename=whatisT18_whatis
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992/