Scientific background
Down syndrome is caused by an additional chromosome 21 (trisomy 21). Life expectancy has increased significantly in recent years, but is reduced by about 20 years compared to the general population. Clinical symptoms include intellectual disability, typical facial features (epicanthus, upward slanting eyes, macroglossia), cardiac abnormalities, muscle hypotonia, sandal gap, and a single transverse palmar crease. Females with Down syndrome are fertile, whereas males are usually infertile. An increased susceptibility to infections and a slightly increased risk of childhood leukemia are characteristic of Down syndrome.
There are very good support options available for those with Down syndrome to help them achieve a high degree of independence in adulthood. Nevertheless, most adults with Down syndrome are reliant on support in their daily lives. Down syndrome occurs with an average incidence of 1:650 newborns, although there is a maternal age effect (incidence 1:1,250 in a 20-year-old woman and 1:90 in a 40-year-old woman). In 92% an additional chromosome 21 is present (free trisomy 21), 3% show a mosaic trisomy 21 and in 5% a so-called Robertsonian translocation between a third chromosome 21 and an acrocentric chromosome is found (so-called hereditary variant of Down syndrome). Partial trisomies due to other translocations are very rare.
References
Antonorakis et al. 2004, Nature Rev Genet 5:725 / Stripoli et al. 1999, Genomics 64:252 / Yamakawa et al. 1998, Hum Molec Genet 7:227 / Baird and Sadovnick 1988, Lancet 2:1354 / Pellisier et al. 1988, Hum Genet 80: 277