Down syndrome/Trisomy 21

Down syndrome (DS), also called Trisomy 21, is a condition in which extra genetic material causes delays in the way a child develops, both mentally and physically. It affects about 1 in every 800 babies. Trisomy 21 is caused by a meiotic nondisjunction event.

Normally, a baby inherits genetic information from its parents in the form of 46 chromosomes: 23 from the mother and 23 from the father.

With meiotic nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. It's this extra genetic material that causes the physical features and developmental delays associated with DS.

Kids with Down syndrome tend to share certain physical features such as a flat facial profile, an upward slant to the eyes, small ears,and a protruding tongue with mild to moderate intellectual impairment.

Scientists found that women age 35 and older have a significantly higher risk of having a child with the condition. At age 30, for example, a woman has about a 1 in 900 chance of conceiving a child with DS. The probability increases to about 1 in 350 by age 35. By 40 the risk rises to about 1 in 100.

For genetic testing before birth, pregnant women may decide to undergo amniocentesis or chorionic villus sampling (refer to Blog on Prenatal testing for Down Syndrome).

Example of Meiotic Nondisjunction that causes disease like Down Syndrome