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Chromosomal Abnormalities: Trisomy 21

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Down syndrome and chromosomal abnormality trisomy 21 iStockphoto/Thinkstock

Trisomy 21, the most common type of Down syndrome, is a chromosomal abnormality where the developing embryo receives three chromosome 21s instead of the usual two.

Normally, a baby inherits one copy of chromosome 21 from his mother’s egg and one copy from his father’s sperm.

In Trisomy 21, one of the gametes (egg or sperm) has two copies so the baby ends up with too much genetic information. This extra genetic information results in varying degrees of physical and intellectual disability.

Trisomy 21 affects 94 percent of those with Down syndrome. There are two other types, translocated Down syndrome, where the extra chromosome 21 is attached to another chromosome (this accounts for 4 percent of cases) and mosaic Down syndrome, where only some cells in the body have the extra chromosome 21. This is the rarest kind, affecting only 2 percent.

Down syndrome was named after a British doctor called John Langdon Down, who described the syndrome in 1866. However, it had already been described by another doctor, Jean Etienne Dominique Esquirol in 1838.

Signs of Down Syndrome

Signs of Down Syndrome include:

• Characteristic facial features like a flat nose, upward slanting eyes, white spots on the iris and abnormally shaped ears

• A small mouth that makes the tongue look too big – sometimes the tongue will protrude

• Smaller than average fingers with a little finger that curves inwards

• Smaller than average feet and a big space between the big toe and the second toe on each foot

• Deep creases in the palms of the hands

• Poor muscle tone and loose ligaments resulting in floppiness

• Lower than average birth size and weight

Children with Down syndrome often have other co-existing medical conditions such as congenital heart disease, hearing problems, eyesight problems, celiac disease, skeletal abnormalities, thyroid dysfunction and dementia.

Some children have gastrointestinal and immune system problems. Around 10 percent of children with Down syndrome also have autism.

Some babies have difficulty with feeding because their mouth and tongue shape are different from normal.

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