Down syndrome or Down's syndrome, is a congenital condition that usually causes some degree of learning disability and is typically characterized by certain physical features. In the UK, approximately 750 babies are born with Down syndrome every year and the condition affects individuals of all ethnicities.
Some examples of the typical features seen in people born with Down syndrome include:
- Reduced muscle tone or hypotonia
- Eyes that slant upwards and a flat nasal bridge
- A single crease across the palm known as the palmar crease
- Low birth weight
- Delayed ability to crawl and walk
- Some degree of learning disability
Although many individuals with Down syndrome share characteristic features of the condition, they look different to each other and usually resemble their mother, father or other family members.
The level of learning disability also varies between individuals, with some only experiencing mild intellectual disability (IQ 50 to 70) and others experiencing severe difficulties (IQ 20 to 35).
Down syndrome is caused by the presence of an additional copy of chromosome 21 in a person’s cells. This extra copy is acquired by chance and although Down syndrome is more common among babies born to mothers of an older age, mothers of any age may have a baby with the condition.
Humans usually have 46 chromosomes in every cell, with 23 inherited from each parent. Due to the extra copy of chromosome 21, people with Down syndrome have 47 chromosomes in their cells. This additional DNA causes the physical characteristics and developmental problems associated with the syndrome.
Down syndrome is an incurable condition but there are many children with this condition who grow up to lead fulfilling lives. Down syndrome can be identified before birth using antenatal screening and further tests to confirm the likelihood of a child being born with the condition. To detect the likelihood of Down syndrome, the number and appearance of chromosomes present in the nucleus are assessed using a process called karyotyping.