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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 II

This individual is a female: note the two X chromosomes and the lack of a Y chromosome. This individual carries three copies of chromosome 13, a chromosomal aneuploidy called Trisomy 13 syndrome or Patau syndrome. The syndrome is caused by the presence of an extra copy of chromosome 13. This karyotype would be abbreviated 47,XX,+13.

Babies with Trisomy 13 can have a variety of congenital abnormalities including incomplete development of the forebrain, olfactory and optic nerves, seizures, apnea, mental retardation, deafness, microcephaly (small head), sloping forehead, cleft lip and/or cleft palate, skin abnormalities, congenital heart defects, additional fingers and toes, prominence of the heels of the feet, thin or missing ribs, and other abnormalities.

Trisomy 13 syndrome occurs in approximately 1 of every 5,000 live births. More than 95% of fetuses that are trisomic for chromosome 13 spontaneously abort during gestation. Most babies with Trisomy 13 that are born alive die within the first few months of life.

About 20% of cases of Trisomy 13 are caused by a translocation, that is when all or a portion of chromosome 13 is attached to another chromosome (often chromosome 14). More often, the extra copy of chromosome 13 is 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 Trisomy 13 syndrome occur during maternal meiosis. 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 Trisomy 13 syndrome being born into a family.