Down syndrome
Down syndrome is a chromosomal condition caused by an extra copy of chromosome 21, leading to intellectual disability, characteristic facial features, and an increased risk for various health complications.
Overview
Down syndrome is associated with mild to moderate intellectual disability, developmental delays, and hypotonia (weak muscle tone) in infancy. While cognitive delays are universal among affected individuals, the severity varies.
Physical and Medical Characteristics
Common physical features include a flattened face, upward-slanting eyes, small ears, and a tongue that tends to stick out. Medical risks include:
- Congenital heart defects (occurring in about half of affected children).
- Digestive abnormalities, such as intestinal blockage.
- Gastroesophageal reflux and celiac disease.
- Hypothyroidism (underactive thyroid gland).
- Hearing and vision problems.
- Increased risk of leukemia (cancer of blood-forming cells).
Behavioral and Cognitive Development
Children with Down syndrome often experience delayed speech and language development. Behavioral issues may include attention problems, obsessive/compulsive behavior, and stubbornness. A small percentage may be diagnosed with autism spectrum disorders. As adults, individuals have an increased risk of developing Alzheimer's disease, often in their fifties or sixties.
Causes
Down syndrome is typically caused by trisomy 21, where each cell contains three copies of chromosome 21 instead of two. Less common causes include:
- Translocation Down syndrome: Part of chromosome 21 is attached to another chromosome.
- Mosaic Down syndrome: Only some of the body's cells contain an extra copy of chromosome 21.
Inheritance
Most cases of trisomy 21 and mosaic Down syndrome are not inherited and occur as random errors in cell division. Translocation Down syndrome can be inherited from an unaffected parent who carries a balanced translocation, which can become unbalanced when passed to the next generation.