What Happens if You Have Three of Chromosome 21?

Trisomy 21 or Down syndrome is one of the most well-known genetic disorders. This condition results from what geneticists call chromosomal nondisjunction, when two chromosomes fail to separate during the process that makes sperm and egg cells. Although Down syndrome causes a variety of health and developmental problems, given good medical care there's no reason why people with Down syndrome can't enjoy a long, fulfilling life.

Meiosis

• Human gonads produce sperm and egg cells through a process of cell division called meiosis. Meiosis has two separate phases, meiosis I and meiosis II. Ordinarily sperm and egg cells have one copy of all 23 chromosomes; if chromosomal nondisjunction occurs during either phase, however, it creates a sperm or egg cell that has two copies of a given chromosome instead of one. When this egg is fertilized (or when this sperm cell fertilizes an egg), it will create a zygote that has three copies of one chromosome and two copies of all the rest. If this happens with chromosome 21, the condition is called Down syndrome.

Effects

• If your child has Down syndrome, she'll typically have flattened facial features and a small head on a short neck. Her eyes will slant upwards and her ears will be unusually shaped. While babies with Down syndrome are often no different in size from other babies, they grow slowly and children with the condition are generally shorter than other children the same age. People with Down syndrome usually exhibit mental retardation in the mild to moderate range; the precise degree of mental retardation varies between individuals.

Medical Conditions

• If you have Down syndrome, you may be at increased risk for various medical conditions. Heart defects, leukemia, and thyroid problems are a few of the health problems more common among Down syndrome patients than the general population. With appropriate medical care, however, people who have Down syndrome can often live to their 50s and 60s or beyond. Nor does Down syndrome preclude an active and fulfilling life. People with Down syndrome can read and write, attend mainstream schools, hold jobs, contribute to their communities and exhibit a diverse variety of talents and interests just like anyone else.

Aneuploidy

• Down syndrome is an example of a class of genetic disorders called aneuploidy, all of which involve an aberrant number of one of the 23 chromosomes. For reasons not yet fully understood, aneuploidy is unusually common in humans. According to "Genetics," current evidence suggests some 30 percent of conceptions miscarry--often so early on the mother is unaware of her pregnancy. 50 percent of spontaneously aborted fetuses exhibit chromosomal defects, and aneuploidy is by far the most common of these. This high incidence is striking when you consider that in mice, your fellow mammals, a mere 2 percent of all fertilized eggs exhibit aneuploidy. Aneuploidy in humans is typically lethal; of all fetuses with a chromosomal defect, only 2 percent will survive to birth. Down syndrome is one of several notable exceptions.

Incidence

• About 400,000 people in the U.S. live with Down syndrome, and the condition occurs in about 1 in 700 births. Incidence increases with the age of the mother, rising to 1 in 100 for women at age 40. While Down syndrome is typically spontaneous, in some cases it has a tendency to run in families; this condition is called familial Down syndrome. It results from a type of genetic alteration called a Robertsonian translocation, where the short arm of chromosome 14 has been swapped for the long arm of chromosome 21. This condition only accounts for 4 percent of all cases.