What is Down syndrome?

Down Syndrome is caused by changes to the 21st chromosome that occur during cell division in the early stages of pregnancy.

Normal Chromosome Pattern
Normal Chromosome Pattern

Three genetic variations can lead to Down syndrome. The 21st chromosome does not divide but instead remains whole, causing an 'extra copy' of itself (non-disjunction). Or part of the 21st chromosome can break away (translocation) and join another chromosome, creating repeated genetic material. Sometimes there is a mix of cell lines, some of which have a normal set of chromosomes and another set, which have the extra copy of chromosome 21. This form of Down syndrome is called Mosaicism.

 Example Trisomy 21 Pattern
Example Trisomy 21 Pattern

When the changes to the chromosome occur they affect DNA (our genetic makeup) and our cells, this leads to various changes throughout the bodily systems. Although there are varying degrees of change, they usually result in delayed development in cognitive, pathological and physical aspects; this means development is slower than what might be considered 'normal' for the average child.

The change to the chromosomes may also bring out into the open any 'weaknesses' that might be present in cell lines and so children with Down syndrome may be more susceptible to ailments that are already within the general public, such as thyroid dysfunction, diabetes and heart conditions as well as decreased hearing and poorer vision. 

The types of Down syndrome, as mentioned above, only tell us how the genetic change occurred; in each case the resulting condition can range from mild to severe.


Approximately 92% of the time, the most common form of Down syndrome is caused by the presence of an extra chromosome 21 in all cells of the individual. In such cases, the extra chromosome originates in the development of either the egg or the sperm. Consequently, when the egg and sperm unite to form the fertilized egg, three--rather than two--chromosomes 21 are present. As the embryo develops, the extra chromosome is repeated in every cell. This condition, in which three copies of chromosome 21 are present in all cells of the individual, is now often referred to medically as Trisomy 21


Approximately 3-4% of individuals with Down syndrome have cells containing 46 chromosomes and not 47, which is the most typical representation of the extra copy of chromosome 21. These individuals still have the features associated with Down syndrome because the material from one chromosome 21 gets stuck or translocated (moved) onto another chromosome. As there is still much genetic material from chromosome21, it still results in the traits associated with Down syndrome, although the normal amount of Chromosomes. This Variation of Down syndrome is referred to medically as Translocation Trisomy 21


In approximately 2-4% of cases, Down syndrome is due to Mosaic Trisomy 21. In this instance, the extra chromosome 21 is present in some, but not all, cells of the individual. For example, the fertilized egg may have the right number of chromosomes, but, due to an error in chromosome division early in embryonic development, some cells acquire an extra chromosome 21. Thus, an individual with Down syndrome due to mosaic trisomy 21 will typically have 46 chromosomes in some cells, but will have 47 chromosomes (including an extra chromosome 21) in others. In this situation, the range of the physical problems may be less depending on the proportion of cells that carry the additional chromosome 21.

Health Notes

Due to the more than usual amount of health problems caused by Down syndrome, many children are often born with heart abnormalities, some may clear up on their own in time and some may require surgery urgently or at a later date.

To this end it is important to make sure all follow-up cardiac appointments are attended.

Regular appointments should also be made for E.N.T and vision.

A thyroid test should be done after the birth and thereafter yearly. If your child sweats profusely or has difficulty maintaining body temperature it is always sensible to check with your Doctor.

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