Down syndrome is a chromosomal condition caused by an extra copy of chromosome 21. This additional genetic material alters development, leading to the characteristics associated with the syndrome. The condition is not related to race, nationality, or socioeconomic status, occurring in people of all backgrounds globally.
The Genetic Basis of Down Syndrome
Human cells contain 46 chromosomes arranged in 23 pairs. Down syndrome occurs from the presence of a third copy of chromosome 21, which causes the condition’s associated developmental and physical features. The specific way this extra chromosome appears allows for classification into three types.
The most common form, Trisomy 21, accounts for about 95% of cases. It occurs from an error in cell division called nondisjunction, where the 21st chromosome pair in the sperm or egg fails to separate. As the embryo develops, this extra chromosome is replicated in every cell.
Translocation Down syndrome makes up about 3% of cases. This type happens when an extra part or a whole chromosome 21 attaches to another chromosome. Mosaic Down syndrome accounts for roughly 2% of cases and occurs when an individual has a mixture of cell types. Some cells have the typical 46 chromosomes, while others contain the extra chromosome 21.
Common Physical Characteristics
People with Down syndrome often share certain physical traits that are identifiable at birth. These characteristics result from the extra chromosome and are present regardless of racial background, appearing alongside a person’s inherited familial features.
Common traits include a flattened facial profile, especially across the bridge of the nose. The eyes may have an upward, almond-shaped slant due to a skin fold on the upper eyelid. Some individuals also have small white spots on the iris, known as Brushfield spots.
Other signs can include a shorter neck, smaller ears, and a small mouth. Low muscle tone (hypotonia) at birth is also frequent, as is a single deep crease across the palm, known as a palmar crease. An individual may have these characteristics to varying degrees, and some may not be present at all.
Prevalence and Maternal Age
Down syndrome is the most common chromosomal condition in the United States, occurring in about 1 in every 640 babies. Data suggests a higher prevalence in Hispanic infants, followed by white infants, and lowest in Black infants. These differences may be influenced by the average maternal age within these groups and disparities in prenatal screening.
Maternal age is a primary factor influencing the chance of having a baby with Down syndrome. As a woman’s age increases, so does the risk of the chromosomal error causing Trisomy 21. A 35-year-old woman has about a 1 in 350 chance, which rises to 1 in 100 by age 40 and 1 in 30 by age 45.
Despite the increased probability in older women, most babies with Down syndrome are born to mothers under 35, as this group has higher birth rates. The additional chromosome can be traced to the father in about 5% of cases.
Associated Health Considerations
People with Down syndrome have an increased likelihood of certain health conditions, though not everyone develops them. Regular medical care is used for monitoring and management, as many of these conditions are treatable with early detection.
Congenital heart defects are a primary health concern, affecting nearly half of all infants with the condition. These can range from minor issues to serious defects requiring surgery. Due to this high frequency, all newborns with Down syndrome are screened for heart conditions.
Other common health considerations include:
- Thyroid problems, particularly an underactive thyroid (hypothyroidism).
- Increased susceptibility to infections.
- Hearing loss, which may affect up to 75% of individuals.
- Vision problems, including eye diseases like cataracts, affecting about 60%.
- Gastrointestinal issues and blood disorders such as leukemia.
- A higher risk of developing Alzheimer’s disease later in life.
Life, Development, and Support
People with Down syndrome have a wide spectrum of abilities and lead full lives. Improved understanding and supportive services have changed outcomes over recent decades. Today, the average life expectancy is around 60 years, a major increase from 25 years in the 1980s.
Early intervention programs help children maximize their potential, often starting shortly after birth. These services include physical, occupational, and speech therapies. Physical therapy focuses on motor skills and muscle strength, while occupational therapy assists with daily living activities. Speech therapy addresses language development.
Advances in educational and social inclusion allow individuals to attend school, participate in community activities, and form meaningful relationships. With proper support, many adults with Down syndrome hold jobs and live with a degree of independence. These improvements in medical care and social integration have greatly improved quality of life.