The genetics of Down syndrome

Down syndrome is a specific chromosomal disorder resulting from the presence of an extra chromosome. Chromosomes are structures that contain the genetic information people need to grow and develop. They are present in all the cells in our bodies.

Each chromosome is made up of DNA, which contains encoded genetic instructions (genes) for the development of all the structures and functions in the body. Every chromosome contains thousands of genes. Our development is precisely controlled by our genes, so that if a person has either too much or too little chromosomal material this can have a significant effect on their development.

Each cell in the human body usually contains 46 chromosomes arranged in 23 pairs, which are labelled 1-23. For example, the 23rd pair are the so-called 'sex chromosomes' that determine whether a baby is a boy or a girl. In people with Down syndrome, an extra copy of chromosome 21 is present.

There are three forms of Down syndrome:

  1. Trisomy 21 Ninety-five percent of people with Down syndrome have trisomy 21. In this type of Down syndrome, every cell in the body has an extra chromosome 21.
  2. Mosaic Down syndrome One to two percent of people with Down syndrome have mosaic Down syndrome. In this type of Down syndrome, only some cells have the extra chromosome 21. The rest of the cells have the usual genetic composition. This sometimes leads to a milder level of intellectual disability and less obvious physical features of Down syndrome.
  3. Translocation Down syndrome Three to four percent of people with Down syndrome have translocation Down syndrome. In this type of Down syndrome, extra chromosome 21 material is attached - or translocated - to a different chromosome. This variation does not significantly change the effect of the Down syndrome. Translocation Down syndrome is sometimes hereditary.

The type of Down syndrome is identified by a blood test, usually taken shortly after birth.

Although the chance of having a baby with Down syndrome increases with maternal age, children with Down syndrome are born to mothers of all ages. Most babies with Down syndrome are born to mothers under 35 years of age, because this is the group to which the greatest number of babies are born overall.

Screening and diagnostic tests for Down syndrome are available before a baby is born. The following fact sheets may be helpful:

Down Syndrome Australia's, Testing for Down syndrome - Fact Sheet
VCGS Prenatal testing -
Fact Sheet

For more information about test types, costs or understanding results contact your doctor or Victorian Clinical Genetic Service Victoria