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 normally has 23 pairs of chromosomes, meaning that there are 46
chromosomes in total. A person with trisomy has 47 chromosomes, since there is one
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
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-
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?
The most common type of trisomy is Down syndrome, in which there is an extra 21st chromosome. For
this reason, Down syndrome is also known as trisomy 21. Down 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
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 (shown in the picture above) is
known as Edward's syndrome. Edwards syndrome is a disorder characterized by mental retardation,
abnormal skull shape, abnormally shaped ears 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. Trisomic means relating to trisomy.
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."