Super rare chromosomes

2020 - Day 2
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Super rare chromosomes. Day 2, 2020

Hi, I’m Kieren and I am four years old. I live with my mum, dad and older sister in Victoria. When my mum was pregnant with me, I was diagnosed with a rare variant of Down syndrome known as mosaic Trisomy 21.

Most people with Down syndrome have three copies of the 21st chromosome in every cell.

Kieren Smiling

But only some of my cells have a third copy of the chromosome 21. The rest of my cells have the usual two copies. Roughly 2% of people diagnosed with Down syndrome have this variant.

But the surprises did not stop there!

After I was born, my blood test showed that the arrangement of my 21st chromosome is super rare. I have what is known as an isochromosome.


Chromosomes are thread-like structures in the cells that hold the genes.  They have two parts: a long arm known as the ‘q arm’ and a short arm known as the ‘p arm’.

At some point in the early days of my development before I was born, I ended up with three copies of the long q arm of my 21st chromosome. Two of these fused together and the other one lost its p arm. So, I have a mosaic, isochromosome arrangement of my 21st chromosome which is incredibly rare!

Today I am a happy, energetic and busy little kid. I talk a lot, I love imaginative play, and the beach is always my happy place. I adore going to kinder and making new friends wherever we go.

(written by Kieren’s mum Carolyn Martin)

Find out more about isochromosomes on Wikipedia.

Image credit: Pathogenetics. An introductory review – Scientific Figure on ResearchGate. Available from: [accessed 1 Feb, 2020]. Licence: (CC By-NC-ND 4.0)