What is a Trisomy? - Rainbows Down Under

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. Under normal circumstances every human has 46 chromosomes in each cell of their body. A trisomy occurs when there are 47 chromosomes instead of the usual 46.

There are different types of trisomy, for example: a partial trisomy or trisomy mosaicism Unless it is otherwise indicated if I refer to a "trisomy" I am referring to a whole additional chromosome that appears to be in every cell of the body. A partial trisomy occurs when part of an extra chromosome, as opposed to a whole extra chromosome, occurs. In this case the additional chromosome is usually attached to one of the other chromosomes. Trisomy mosaicism is where not every cell contains the extra chromosomal material. I will explain these different types of variations more completely later.

A trisomy can occur with any of the chromosomes however the most commonly known trisomic conditions are: Trisomy 21 (Down Syndrome) Trisomy 18 (Edward's Syndrome) Trisomy 13 (Patau's Syndrome)

How does a Trisomy happen? What does it mean? To understand these questions we will need to look at some basic genetics. By clicking on the next buttons I will try and answer some of these questions as simply as I can.

I do ask however that you bear in mind that I am not a doctor and the information contained in this site is my interpretation of material that I have come across. In no way will it substitute for good medical care, understanding and an explanation of your own particular circumstances. Please use it as it is intended, as a starting point for discussion with your doctors. To make informed choices you need INFORMATION.


Rainbows Down Under	What is a chromosome?


What is a Trisomy Index
What is a chromosome?		The human egg & sperm cell
How does it happen?		What is "full" trisomy?
What is a partial trisomy?		What is a translocation?
What is mosaicism?		What is a karyotype?
What is a Phenotype?		What is prenatal testing?
How is a diagnosis made?		Will my baby die?
Why did this happen to me?		What are the risks of it happening again?

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What is a Trisomy Index