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FEATURED BOOKS ON GENETIC DISORDERS FROM BARNES & NOBLE.COM:
WHAT IS TRISOMY?
Trisomy is the presence, within a person's cells, of an extra set of chromosomes so that there are three chromosomes of a certain number instead of the usual two. Chromsomes are structures that contain genes. Genes are units of material contained in a person's cells that contain coded instructions for how certain bodily characteristics (such as eye color) will develop. All of a person's genes come from his/her parents.
TELL ME MORE ABOUT CHROMOSOMES?
Each person has 23 pairs of chromosomes, meaning that there are 46 chromosomes in total. A person with trisomy has 47 chromosomes, since there is one extra chromosome. One of each pair of chromosomes is inherited from the mother and one of each pair is inherited from the father. The first 22 pairs of chromosomes (known as autosomes) are not involved in determining sex. The 23rd pair of chromosomes, however, is involved in determining sex.
The 23rd pair of chromosomes consists of X and/or Y chromosomes. An X chromosome is shaped like and X, whereas a Y chromosome is shaped like a Y. If a person has two X chromosomes, the person will develop as a female. If a person has an X and a Y chromosome, the person will develop as a male.
WHAT KIND OF PROBLEMS CAN RESULT FROM TRISOMY?
Trisomy can result in a miscarriage, in which the embryo dies inside the pregnant mother. With regards to humans, an embryo is a very early form of the person from about 2 weeks after it was conceived until the end of the 7th or 8th week. Trisomy can also result in a wide range of abnormalities in a live-born child.
WHAT CAUSES TRISOMY?
Trisomy can result from a fault during the formation of an egg or sperm (the male sex cell), in which an extra chromosome gets into the cell. An egg and a sperm come together to make an embryo (see last section). If an affected egg comes together with sperm or if affected sperm comes into contact with an egg, the resulting embryo will have an extra chromosome, causing trisomy. The cell that contains 47 chromosomes is labeled in medicine as 2n +1. 2n+1 means there are 2 complete sets of normal chromosomes, plus one extra chromosome.
Trisomy can also be caused by a rearrangement of the chromosomes inside the person's cells. This is known as translocation and it is inherited from one of the parents. Older mothers are more likely to have children with trisomy.
WHAT ARE SOME EXAMPLES OF TRISOMY?
An extra 13th chromosome is known as Patau's syndrome. Patau's syndrome is a disorder characterized by mental retardation, abnormally formed ears, a small jaw, heart defects, abnormal flexing of the fingers, an extra finger or an extra toe, abnormally rotated intestines, kidney defects, and other abnormalities. The intestine is a tube shaped structure that is part of the digestive tract. It stretches from an opening in the stomach to the anus (the area that poop comes out of) and occupies most of the lower parts of the belly. The kidneys are two organs located on each side of the spine, behind the stomach. The kidneys filter (remove) wastes from the blood. Patau's syndrome is also known as trisomy 13. An extra 18th chromosome is known as Edward's syndrome. Edwards syndrome is a disorder characterized by mental retardation, abnormal skull shape, abnormally shaped ears that are positioned low on the head, heart defects, a small jaw, abnormal flexing of the fingers, and other abnormalities. Edward's syndrome is also known as trisomy 18.
Other types of trisomies, such as trisomy 8 and trisomy 22, are extremely rare. In trisomy 8, there is an extra 8th chromosome. In trisomy 22, there is an extra 22nd chromosome. Trisomies can occur with any of the 23 pairs of chromosomes, but they mostly affect the smaller chromosomes or the sex chromosomes (chromosome pair 23).
Although trisomies are often referred to by the number of the chromosome pair where there is an extra chromosome (for example, trisomy 13), trisomies can also be labeled by group. There are 7 groups of chromosomes, labeled A through G. For example, chromosomes 21 and 22 are located in group G. Therefore, trisomy 21 (Down's syndrome) may also be referred to as trisomy G.
WHAT IS THE DIFFERENCE BETWEEN A FULL TRISOMY & A PARTIAL TRISOMY?
In a full trisomy, there is an entirely complete extra chromosome. In a partial trisomy, there is only part of an extra chromosome present. All full trisomies cause multiple abnormalities, such as bone and heart defects, mental impairment, and facial abnormalities. Partial trisomies have variable effects, depending on how much extra chromosome material is present.
HOW ARE TRISOMIES DIAGNOSED?
Trisomies are diagnosed by analyzing a person's chromosomes. This can be done through a blood test and is known as chromosomal analysis.
IS THERE ANY TREATMENT FOR TRISOMY?
There is no specific treatment for trisomy disorders. Although many children with Down's syndrome survive into adulthood, many children with other types of trisomy (such as trisomy 13 and trisomy 18) die by age 2 or 3. Some trisomies (such as trisomy 11) have never been seen in live births. These types of trisomies lead to an early death of the unborn child or can lead to a stillbirth (when the baby is born dead).
Parents of an affected baby should seek medical advice from a doctor that specializes in genetics (the study of genes) to assess the chances of a future child being affected. This type of advice is often referred to as genetic counseling.
WHAT ELSE IS TRISOMY KNOWN AS?
Trisomy is also known as trisomia.
The most common type of trisomy is Down's syndrome, in which there is an extra 21st chromosome. For this reason, Down's syndrome is also known as trisomy 21. Down's syndrome is an abnormality that is present from birth that results in mental impairments and a characteristic physical appearance (small facial features, large tongue that sticks out, flatness in the back of the head, and hands that are short and broad).
WHAT IS THE ORIGIN OF THE TERM, TRISOMY?
Trisomy comes from the Latin word "tri" meaning "three," and the word part "some" from the word "chromosome." Put the words together and you have "three chromosomes."
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