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Archive for the ‘Genetics’ Category

Down syndrome is usually diagnosed at birth on the basis of the typical facial features, hypotonia, and single palmar crease. Several serious problems that may be evident at birth or may develop early in childhood include duodenal atresia, congenital heart disease (especially atrioventricular canal defects), and leukemia. The intestinal and cardiac anomalies usually respond to surgery, and the leukemia generally responds to conservative management. Intelligence varies across a wide spectrum. Many people with Down syndrome do well in sheltered workshops and group homes, but few achieve full independence in adulthood.

An Alzheimer-like dementia usually becomes evident in the fourth or fifth decade and, for those who survive childhood, accounts for a reduced life expectancy. Studies addressing the risk and severity of dementia in relation to the apolipoprotein E genotype have had conflicting results. Cytogenetic analysis should always be performed—even though most patients will have simple trisomy for chromosome 21—to detect unbalanced translocations; such patients may have a parent with a balanced translocation, and there will be a substantial recurrence risk of Down syndrome in future offspring.

The presence of a fetus with Down syndrome can be detected in the early second trimester through screening maternal serum for -fetoprotein and certain hormones (“multiple marker screening”) and by detecting increased nuchal thickness on fetal ultrasound. The risk of bearing a child with Down syndrome increases exponentially with the age of the mother at conception and begins a marked rise after age 35. By age 45 years, a mother has one chance in 40 of having an affected child. The risk of other conditions associated with trisomy also increases, because of the increased predisposition of older oocytes to nondisjunction during meiosis.

There is little risk of trisomy associated with increased paternal age. However, older men do have an increased risk of fathering a child with a new autosomal dominant condition. But because there are so many distinct conditions, the chance of fathering an offspring with any given one is extremely small.