Review Note

The symptoms of Down syndrome include intellectual disability (usually mild to moderate); hypotonia (decreased muscle tone); a short stocky build; a wide face, thick tongue, and almond-shaped eyes; developmental delays; and an elevated risk for vision and hearing problems, heart defects, hypothyroidism, and Alzheimer’s disease. 

With regard to etiology, trisomy 21 and mosaic trisomy 21 are both caused by an error during cell division. Also, older maternal age increases the risk of having a baby with trisomy 21 and possibly mosaic trisomy 21, with the risk increasing sharply after 30 years of age. (Research on paternal age and risk for Down syndrome has produced inconsistent results.) 

In contrast, the risk for translocation trisomy 21 is not affected by maternal age, and it can be due to an error during cell division or can be inherited from a parent carrier: When a parent is a carrier and has one child with translocation trisomy 21, there’s a risk that the parent will have another child with this disorder. In addition, relatives of a person who carries the translocation may also have the translocation and be at increased risk of having children with this disorder (Nussbaum, McInnes, & Willard, 2007).