Variable and often mild intellectual disability, almost constant muscular hypotonia and joint laxity are often associated with morphological signs, malformations and risks of complications. Morphological features (upslanting palpebral fissures, epicanthus, flat neck, round face, small nose, bilateral single palmar crease) can be mild and do not constitute a hallmark of the condition. The main potential malformations and complications include: short stature, congenital cataract, conductive hearing loss, heart defects (atrio-ventricular canal), digestive malformations (duodenal atresia), Hirschsprung disease, West syndrome (see these terms), seizures, sleep apnea, sensory deficiencies, leukemia, auto-immune and endocrine pathologies (hypothyroidism, celiac disease (see this term), diabetes mellitus type 1, alopecia areata, earlier aging and Alzheimer disease.
In 95% of the cases, trisomy 21 is "free": the extra chromosome is due to an accidental non-disjunction during meiosis. 2-3% of those cases are in a mosaic state. In the remaining 5%, the supernumerary chromosome 21 or portion of chromosome 21 is translocated to another chromosome.
