Other Names: Trisomy 21, translocation Down syndrome, mosaic Down syndrome
Down syndrome is a common chromosome condition with characteristic facial features and intellectual disability. It is caused by an extra chromosome 21 in all or some cells of the body.
Characteristics of Down syndrome
Down syndrome is the most common chromosome disorder. It occurs in about 1 in every 700 births and affects both males and females. Down syndrome is caused by an extra chromosome 21 in all or some cells of the body. This means that people with Down syndrome have three copies of chromosome 21 instead of the usual two copies. The extra chromosome causes physical and intellectual differences.
People with Down syndrome may have certain facial features including eyes that slant up at the outer corner (upslanting palpebral fissures), skin folds that cover the inner corner of the eye (epicanthic folds), and a flatter mid-face (midface hypoplasia). They may have a single crease across the palm of the hand and often have shorter stature/height. Individuals with Down syndrome may also have congenital heart defects (these occur in about half of people with Down syndrome), difficulty feeding, eye or vision problems, and hearing problems. Affected individuals are usually delayed in meeting developmental milestones such as sitting, walking, and talking. All individuals with Down syndrome have some degree of intellectual disability, usually mild to moderate. The features seen in individuals with Down syndrome are variable and many do not show all the physical or intellectual features associated with the condition.
Down syndrome can be detected by a study that looks at a person’s chromosomes. This can be done through blood tests called a karyotype or FISH study.
Sometimes, Down syndrome can be detected before a child is born. Physical signs of Down syndrome can be seen on prenatal ultrasound in about 50-60% of affected fetuses. Screening for Down syndrome can be done before birth and during pregnancy that estimate the chance that a baby has Down syndrome. Additionally, there are prenatal diagnostic tests (amniocentesis and chorionic villus sampling) available that can determine with near 100% accuracy if a baby has Down syndrome.
Once a diagnosis of Down syndrome is made, there are medical issues to check for and monitor as a child grows. The American Academy of Pediatrics provides guidance for treatment and management of individuals with Down syndrome. Some of the main recommendations include getting a baseline heart ultrasound (echocardiogram), regular eye and hearing exams, regular blood tests that check thyroid function and signs of leukemia, routine dental exams in infancy, a sleep study in toddler years to check for sleep apnea (low breathing or pauses in breathing during sleep), x-rays of the neck and spine, and routine screening for symptoms of celiac disease at all health maintenance visits. It is also important for healthcare providers to discuss early intervention, physical, occupational and speech therapy at early health maintenance visits.
Mode of inheritance
In most cases, Down syndrome is not inherited and is considered a sporadic event with the chance of having a baby with Down syndrome influenced by the mother’s age (discussed below). Sometimes, a person’s chromosomes are rearranged in such a way as to make the chance of having a child with Down syndrome much more likely. Chromosome studies should be performed to determine if someone has one of these chromosome rearrangements and to clarify recurrence risk.
Most common cause:
Approximately 95% of cases of Down syndrome are caused by two copies of chromosome 21 that do not separate properly during the formation of a sex cell (egg or sperm cell). This is called nondisjunction. If a sperm or egg cell with two copies of chromosome 21 combines with a normal sperm or egg cell, the developing embryo then has three copies of chromosome 21, causing Down syndrome. This is also called Trisomy 21. Sometimes the extra chromosome is only in some but not all cells of a person’s body. This is called mosaic Down syndrome or mosaic Trisomy 21. Individuals with mosaic Trisomy 21 may have more subtle features of the syndrome.
Risk to family members
The greatest known risk factor for having a child with Down syndrome is based on maternal age. However, having had a child with Down syndrome also increases the chance of having another child with Down syndrome. As mentioned above, sometimes there are chromosome rearrangements (called a translocation) in an individual that can significantly increase their chance to have children with Down syndrome. A prenatal genetic counselor can help families determine their risks and further explore family planning options.
Genetics Home Reference: Down Syndrome
National Down Syndrome Society
National Down Syndrome Congress
Medical Home Portal: Down Syndrome
Created by:Alizabeth Berg, MS, LGC, Karin M. Dent, MS, LCGC
Edited by:Seema Jamal, MSc, LCGC