Down syndrome is a condition caused by an extra copy of chromosome 21. It is the most common chromosomal disorder, affecting people of all races and economic levels. The condition is associated with a variety of intellectual and physical characteristics, and individuals with Down syndrome possess a wide and diverse range of abilities and talents.
Understanding the Genetic Cause of Down Syndrome
Human cells contain 46 chromosomes, arranged in 23 pairs, which hold the genetic instructions for development and functioning. Down syndrome occurs when an abnormality in cell division results in extra genetic material from chromosome 21 in some or all of the body’s cells.
The most common form, accounting for about 95% of cases, is trisomy 21. It is caused by an error in cell division called nondisjunction. During the formation of reproductive cells (egg or sperm), a pair of 21st chromosomes fails to separate. As a result, a reproductive cell has an extra copy of chromosome 21. When this cell contributes to an embryo’s genetic makeup, the child has 47 chromosomes in each cell instead of 46.
Translocation Down syndrome accounts for about 3-4% of cases. In this form, a portion of chromosome 21 breaks off and attaches to another chromosome, often chromosome 14. While the total number of chromosomes remains 46, the extra part of chromosome 21 causes the characteristics of the condition. This type can be passed down from a parent to a child.
The rarest form is mosaic Down syndrome, affecting 1-2% of individuals. In this variation, the error in cell division occurs after fertilization. This results in a mixture, or mosaic, of cells where some have the typical 46 chromosomes and others have 47. Individuals with mosaic Down syndrome may exhibit fewer of the condition’s characteristics.
Identifying Down Syndrome
Down syndrome can be identified during pregnancy or after a baby is born. Prenatal tests are divided into screening and diagnostic types. Screening tests estimate the probability of the condition but are not definitive. These screenings involve a maternal blood test and an ultrasound to look for markers like nuchal translucency, a fluid-filled space at the back of a baby’s neck.
Cell-free DNA (cfDNA) screening is an advanced blood test that analyzes fetal DNA in the mother’s bloodstream for an increased chance of specific chromosome problems. While highly accurate, it is still considered a screening tool. If a screening test indicates a higher probability, a diagnostic test is offered to confirm the diagnosis.
Diagnostic procedures include chorionic villus sampling (CVS) and amniocentesis. CVS involves taking a small sample of tissue from the placenta, while amniocentesis involves collecting amniotic fluid from the uterus. Both carry a small risk of complications but can provide a definitive diagnosis.
After birth, Down syndrome is often suspected based on the presence of certain physical traits. To confirm the diagnosis, a blood sample is taken from the newborn for a chromosomal karyotype test. This test creates a visual profile of the baby’s chromosomes, allowing a laboratory to confirm the condition.
Common Traits and Associated Health Conditions
Individuals with Down syndrome may have certain shared physical characteristics, though not everyone will have all of them. Muscle tone is often lower, and common features include:
- A flatter facial profile and a small nose
- Upward-slanting eyes
- Small ears and a short neck
- A single deep crease across the center of the palm
Developmentally, individuals with Down syndrome experience delays in reaching milestones like sitting, crawling, and walking. Speech and language development is also commonly delayed. The condition includes an intellectual disability, which usually ranges from mild to moderate. Cognitive development varies significantly from person to person.
Several health conditions are more common in people with Down syndrome, and many are treatable. Congenital heart defects are present in about half of all infants. Later in life, individuals have an increased risk of developing Alzheimer’s disease. Other potential health issues include:
- Hearing and vision problems
- Thyroid conditions
- An increased susceptibility to infections
- Gastrointestinal issues like celiac disease
- A higher risk of developing sleep apnea
Therapies and Support Systems
A range of therapies and support systems help individuals with Down syndrome develop their abilities. Early intervention programs, which begin shortly after birth and continue until age three, are beneficial for addressing developmental delays from a young age.
Physical therapy helps improve motor skills and muscle strength, assisting with milestones like sitting and walking. Speech-language therapy improves communication skills, from early non-verbal communication to developing clearer speech. Occupational therapy helps individuals learn to manage daily tasks like feeding themselves, dressing, and developing fine motor skills.
Educational support is adapted to each student’s learning profile. Many children with Down syndrome are included in typical classrooms with specialized instruction, aides, or other accommodations. These inclusive settings can foster social development and academic learning. Other students may benefit from more specialized educational programs.
Consistent medical care is important for monitoring and managing the health conditions associated with Down syndrome. Regular check-ups with physicians and specialists help to treat issues like heart conditions, thyroid problems, and hearing or vision impairments early. This proactive approach helps individuals maintain their health. The support of family, friends, and community organizations also promotes inclusion and independence.