Dictionary of Human Evolution and Biology

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Down Syndrome

Profound phenotype caused by the presence of three copies of all the genes on chromosome 21 (trisomy 21), a true dosage effect. Clinical features include short stature, a characteristically pleasant facies caused by oblique eyelid openings and a pseudoepicanthic fold, palm creases, a wide and flat skull, spots on the iris, a furrowed tongue, and neck and finger webbing. Down syndrome individuals are at an increased risk for Alzheimer disease, respiratory infections, deafness, and leukemia. Few reach the age of 50 years. A major chromosome anomaly and an example of aneuploidy due, in the majority of cases, to nondisjunction of the maternal 21st chromatids during meiosis I, resulting in an ovum with 24 rather than the normal 23 chromatids. When fused with the gamete from the opposite parent (usually a sperm with 23 chromatids), the affected zygote begins development with 47 chromatids rather than the normal

46. Both males and females may be affected. An analogue of Down syndrome has been observed in higher primates, where the cause is usually trisomy 22. Maternal nondisjunction is responsible for 90% of all cases, with the probability of de novo nondisjunction increasing dramatically after maternal age 35; male nondisjunction accounts for 5%. The translocation form causes the other 5%. In populations where amniocentesis is not routinely available, Down syndrome affects 1: 650 live births. Where genetic counseling is available, incidence is about 1: 1000. Incidence increases dramatically as a function of maternal age such that 1: 16 children born to mothers aged 49 and older have trisomy 21.

See translocation Down syndrome and synteny. Formerly called mongolism; use of this term is now discouraged.

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