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

Dihybrid Cross

Genetic factors of variation

CROSSING OVER DURING MEIOSIS

Prenatal Testing for Down Syndrome

Below are Prenatal tests for Down Syndrome(Trisomy 21)

a) Amniocentesis
This procedure is used to collect amniotic fluid, the liquid that is in the womb. It's performed in the doctor's office or in the hospital on an "out-patient" basis. A needle is inserted through the mother's abdominal wall into the uterus, using ultrasound to guide the needle. Approximately one ounce of fluid is taken for testing. This fluid contains fetal cells that can be examined for chromosome tests. It takes about 2 weeks to determine if the fetus has Down syndrome (47 chromosomes) or not.

b) Chorionic Villus Sampling
In this procedure, instead of amniotic fluid being taken, a small amount of tissue is taken from the young placenta (also called the chorionic layer). These cells contain the fetal chromosomes that can be tested for Down syndrome. The cells can be collected the same way as the amniocentesis, but another method is to insert a tube into the uterus through the vagina. CVS is usually carried out between the 10th and 12th weeks of pregnancy.