How Does a Trisomy Happen? - Rainbows Down Under


Trisomy comes from the Latin word "tri" which means three and "somy" which refers to an autosome, a type of chromosome, but not one of the sex chromosomes, which contain the genetic make-up of a person. So trisomy means that there are three identical chromosomes in a persons cell instead of the usual two

Trisomy 18, also known as Edward's Syndrome, means that there are three 18^th chromosomes. Trisomy 13, also known as Patau's Syndrome or Bartholin-Patau Syndrome , means that there are three 13^th chromosomes, Trisomy 21, also known as Down Syndrome, means that there are three 21^st chromosomes

A trisomy can occur with ANY of the 23 autosomes (chromosomes). Although it is possible for there to be an extra sex chromosome (eg XXX or XYY or XXY) this is not usually referred to as a trisomy but in the case of XXX it is called triple X syndrome. XYY is simply referred to as XYY syndrome and the same with XXY.


A photograph of the "moment of conception" It is thought that the stage has already been set for a trisomy to occur before this point.

In the vast majority of cases a trisomy "happens" before conception. It is thought that either the father's sperm or the mother's egg contained 24 chromosomes instead of 23. When fertilisation occurred the resulting cell or zygote which made up the new baby had 47 chromosomes instead of the normal 46.

This diagram shows conception where there is the usual complement of 23 chromosomes in both the sperm and ovum. This would result in a baby without a trisomy.


*This diagram represents conception where the sperm contains 24 chromosomes instead of the usual 23. This would result in a baby with* a trisomy.**

*This diagram represents conception where the ovum contains 24 chromosomes instead of the usual 23. This would result in a baby with* a trisomy.**

When it comes to chromosomes more genetic material is not necessarily better. It can be likened to adding too much of an ingredient when you are making a cake. Even though it may look all right on the outside, inside it may not taste quite the way the recipe intended. In the case of a trisomy the extra genetic material from the extra chromosome effects every stage of the babies development and growth, including that of the placenta, umbilical cord etc.


The Human Egg & Sperm Cells	What is a "Full" Trisomy?


What is a Trisomy Index
What is a chromosome?		The human egg & sperm cell
How does a trisomy 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