SOFT Australia - What is a Trisomy? Page 2

Trisomy comes from the latin word "tri" which means three. "Somy" refers to an autosome, a type of chromosome (but not one of the sex chromosomes) which contains the genetic makeup of a person..

Put very simply a trisomy is a chromosomal anomaly that is characterised by the presence of an extra chromosome in the cells of a person's (or plant or animal's) body.

When a Trisomy occurs it means that there are three identical chromosomes in a persons cell instead of the normal two.

Trisomy 18 (Edward's Syndrome) means that there are three 18th chromosomes,

Trisomy 13 (Patau's Syndrome) means that there are three 13th chromosomes,

Trisomy 21 (Down Syndrome) means that there are three 21st chromosomes and so on.

A Trisomy can occur with any of the chromosomes. So you can have trisomy 1, trisomy 2, 3, 4, 5 and so on.

	Chromosomes are tiny thread like structures which are contained in the nucleus of every human cell. In general every human cell contains 22 pairs of chromosomes that are known as autosomes and two sex chromosomes that are labelled X or Y. It is these chromosomes which contain the genetic blueprint of life and which cause every person to be a unique individual. Under normal circumstances everyone has a total of 46 chromosomes in each of their cells (including the sex chromosomes), a trisomy occurs where there are 47 chromosomes instead of the usual 46.

Each cell also contains a pair of sex chromosomes. In a male this combination is an X and a Y chromosome (XY). And in a femeale there are two X chromosomes (XX). One X chromosome is inherited from the mother and the other chromosome, be it an X or a Y, is inherited from the father. It is this chromosome, either an X or a Y, inherited from the father, which determines the sex of the baby. Together the 22 pairs of autosomes and the two sex chromosomes make up a total of 46 chromosomes.

There are actually different types of trisomy. When we refer to a trisomy we are normally referring to the situation where every cell in the body contains the extra chromosome. Sometimes, in an effort to distinguish between other types of trisomy this is referred to as "full" trisomy.

	A partial trisomy occurs when there is part of an extra chromosome, as opposed to a whole extra chromosome. Normally this is attached to another chromosome and is often (but not always) the result of one of the parents being the carriers of a balanced translocation. The effects of a partial trisomy on an individual vary from person to person depending on exactly how much of the extra chromosome that is duplicated.
Mosaic trisomy is where not every cell contains the extra chromosomal material. Some cells have the extra chromosome (47) and some cells have the usual compliment (46). The effects of mosaicism vary depending on how many cells are effected (the percentage) and where in the individuals body those cells are (the distribution).

This is a general overview of chromosomal anomalies as they relate to a trisomy. There are many other kinds including deletion syndromes, inversions, balanced translocations, Robertsonian translocations and more.

A trisomy can occur with any of the chromosomes, including the sex chromosomes. However the most commonly known trisomies are:

Trisomy 21 (Down Syndrome)
Trisomy 18 (Edward's Syndrome)
Trisomy 13 (Patau's Syndrome)

Trisomy of the sex chromosomes is often viewed differently from a trisomy of the autosomes because the genetic material contained on the sex chromosomes is very different to that of the autosomes.
	Through research and the development of staining techniques in the 1950's, scientists were for the first time able to view the human chromosome. Although they appear disorganised within the cell, scientists have been able to identify them and so have numbered them from 1 to 22 in order of size. When this is done it is called a karyotype.

While all care has been taken the information contained in this web site is not written by a medical professional and as such should not replace the valuable and personal information of your own health care professionals. SOFT does not promote or recommend any specific therapy, treatment or course of action. SOFT is an independent organisation and does not support any particular political or religious point of view. Individuals or organisations referred to, do not necessarily endorse this publication or the views of SOFT.

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