What is a Partial Trisomy? - Rainbows Down Under


There are actually a number of different types of trisomy. The ones most commonly encountered are Trisomy - sometimes referred to as "full" trisomy Mosaic Trisomy and Partial Trisomy

A partial Trisomy means that there is only a part of a chromosome duplicated as opposed to whole chromosome.

Chromosomes are actually made up of two parts, which are called arms. They are joined by something called a centromere. The centromere in a karyotype is the "pinched" or narrower area in the middle of the chromosome. The centromere is very important in cell division.


This picture is an idiogram (which simply means a drawing) of an 18^th chromosome. It shows the two arms and where the centromere would be.

Scientists have labelled each of the arms of a chromosome. The short arm is called "p" for petite or small. The long arm is called "q" simply because it's the next letter in the alphabet! It is the arms of the chromosomes that contain the genes, or the actual "blue prints". In a karyotype the chromosomes are always arranged so that the p arms are at the top.

The bands of black and white that you can see on diagrams of a chromosome represent the light and dark areas (not black and white) that are visible on a chromosome after it has been stained. These bands provide further definition of the chromosome for the scientists. As a result they are numbered from the centromere. The larger the number the further away from the centromere it is and the closer it is to the end. The end of a chromosome is called the terminus.

	Here is an idiogram of an 18^th chromosome showing the numbering.

As you can see the q arm being longer contains more genes. You would normally think that this means it will cause more problems because there is more information duplicated. Sometimes this is the case, however not all of the genes on our chromosomes are actually "active" and it really does depend on each individual chromosome.
For example, Trisomy 13p (that is a duplication of the p arm of the 13th chromosome) is often considered harmless because the genes contained on the p arm of the 13th chromosome are thought to be "inactive".


If the entire q arm is duplicated, from the centromere to the very end, the trisomy is named accordingly. So in the case of the 18^th chromosome it would be Trisomy 18q (T-18q). Or if the p arm was duplicated from the centromere to the very end it would be Trisomy 18p (T-18p).

Sometimes however only a part of an arm is duplicated. Usually this is from somewhere below (or above) the centromere to the end of the chromosome. Sometimes it can just be a fragment from the middle of a chromosome. Regardless these types of duplications are known as a partial trisomy eg. Partial Trisomy 18q or Partial Trisomy 18p.

On a karyotype partial trisomies usually carry a number with them that indicates the "break points", that is where the extra part of the chromosome begins and ends. This can make the karyotype look very complicated and hard to read.
For example, 46,XY,dup(18)(q11.1q22) actually says that the long arm of the 18th chromosome (q) is duplicated between bands 11.1 through to the 22nd band. The whole q arm of the chromosome is NOT duplicated, there is a piece on the end of the 18th chromosome is not. In lay terms then this would be a partial Trisomy 18q.
Break points, or the band numbers are actually quite important and if your child has a partial trisomy you may want to make sure that you know the break points and the arm which is duplicated. If you are trying to find others with a similar partial trisomy diagnosis, it is essential to know the break points. Remember though that even if the trisomies are identical or similar, each child is a unique individual and no two children express an anomaly the same way.

The part of the chromosome that is duplicated may not necessarily be on its own in the cell. Most commonly it actually attaches itself to another chromosome. For example the extra p arm of an 18^th chromosome could actually be attached to the 9^th chromosome.
Partial trisomies are actually quite complicated and so you might want to ask some extra information from your doctor, that is, where the break points are. Whether it is from the centromere or a band on the chromosome. If it is from a band then it is a good idea to get the number. Also if it is known whether the duplicated genes are "active".
Also often, but certainly not always, partial trisomies reflect something called a translocation.


What is a "full" Trisomy?		What is a Translocation?


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