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Overview of the Respiratory System

Author(s): Nancy Moreno, PhD, Deanne Erdmann, MS, and Sonia Rahmati Clayton, PhD
Showing Results for: human Return to Presentation

Be the Cytogeneticist I

This individual is a female: note the two X chromosomes and the lack of a Y chromosome. This individual carries three copies of chromosome 21. This chromosomal aneuploidy is called Trisomy 21 syndrome, or Down syndrome. Down syndrome is caused by the presence of an extra copy of chromosome 21. This karyotype would be abbreviated 47,XX,+21.

Individuals with Down syndrome can have a variety of abnormalities including mental retardation, speckling of the irises of the eyes, poor muscle tone (hypotonia), characteristic appearance of the face, cardiac abnormalities, loose skin in the back of the neck, and fine, soft hair. Individuals with Down syndrome also are prone to leukemia, premature aging, and Alzheimer's disease.

Down syndrome affects approximately 1 in every 800 live births. Approximately 75% of Down syndrome fetuses spontaneously abort during gestation. Of babies born with Down syndrome, most carry an extra chromosome 21. The extra chromosome most often occurs as the result of an error in cell division during meiosis, the process during which germ cell precursors divide to create eggs and sperm. During meiosis, there are occasional sorting errors wherein the chromosomes do not get properly assorted to the dividing cells. This can result in a sperm or egg with an improper number of chromosomes. Most of the meiotic non-disjunction events that result in Down syndrome occur during maternal meiosis.

In some cases of Down syndrome, however, the extra copy of chromosome 21 is attached to the end of another chromosome. Chromosomal rearrangements in which all or a portion of one chromosome becomes attached to another chromosome are called translocations. Translocation cases account for approximately 3% of Down syndrome cases. The source of the extra chromosomal material, whether it is derived from a parental translocation chromosome or as the result of a meiotic non-disjunction event, is important for estimating the chance of another child with Down syndrome being born in a family.

The risk for a woman to have a child with Down syndrome rises with age, and increases significantly after age 35, a phenomenon called maternal age effect. Despite this maternal age effect, most babies with Down syndrome are born to women under 35, because more babies are born to women under 35 overall, than are born to women over 35.