Down Syndrome FAQ

There are three types of Down syndrome, also known in medical terms as trisomy 21.

The most common form of Down syndrome is called nondisjunction trisomy 21 and is a random genetic condition caused by an extra chromosome. It occurs in approximately one in 700 live births and results when the baby ends up with an extra copy of chromosome 21. People with Down syndrome have 47 chromosomes instead of 46. This extra chromosome can lead to health issues, developmental delays and learning disabilities.

The other forms of Down syndrome are mosaic Down syndrome and translocation Down syndrome.

Mosaic Down syndrome is quite rare (approximately >5% of people with Down syndrome have this) and occurs when not every cell has an extra copy of the 21st chromosome. Often people with mDs can go a long time before being diagnosed as sometimes they do not have the obvious physical characteristics associated with Down syndrome, such as the almond-shaped eyes, a single palmar crease or sandal gap.

The third type of Down syndrome is translocation.

This is the only form of Down syndrome that is inherited. Translocation Down syndrome occurs when one chromosome breaks off and attaches to another chromosome and creates excess genetic material. In this case, there are three 21st chromosomes but one of the 21st chromosomes is attached to another chromosome, generally chromosome 14, 13, 15 or 22. In some cases, two of chromosome 21 can be attached to each other.

Inherited translocation Down syndrome occurs because one of the parents is a balanced carrier. That parent will actually have 45 total chromosomes in each cell of their body, but are typical and healthy because there is no excess genetic material.

This parent still has only two copies of each chromosome, but two of these chromosomes are attached to each other. There is also non-inherited translocation as described above, but this is a completely random event and neither parent is a balanced carrier.