In the cell, genetic material is contained in discrete units of deoxyribonucleic acid (DNA) called chromosomes. Normal human cells contain 23 pairs of chromosomes - 22 pairs of non-sex chromosomes (autosomes) and one pair of sex chromosomes. Trisomies are genetic abnormalities where instead of a normal pair of chromosomes, three copies of a chromosome are present. Trisomies can occur in either sex chromosomes or autosomes.
Trisomy 13 is a severe and often lethal genetic disorder that occurs in individuals who carry three copies of chromosome 13 in their cells. Trisomy 13 is the third most common autosomal trisomy in newborns, after Down's syndrome (trisomy 21) and Edwards' syndrome (trisomy 18). It is estimated that trisomy 13 occurs in 0.02-0.005% of newborns, or one out of every 5,000-20,000 live births.
Thomas Bartholin first described the developmental defects that are typical of trisomy 13 in 1656. However, the underlying genetic cause was not discovered until 1960, when Klaus Patau characterized the trisomic nature of the disease. Thus, trisomy 13 is sometimes called Patau syndrome, Bartholin-Patau syndrome, or 13+ syndrome.
A significant proportion of trisomy 13 pregnancies end in spontaneous abortion (miscarriage) or stillbirth. Of the trisomy 13 live births, the median survival time is 7-10 days, and the vast majority (90-95%) of affected patients die within one year. Survival times of up to 10 years have been reported for trisomy 13 patients, although these cases are extremely rare.
The presence of an extra chromosome often results in death before birth, usually in the early stages of pregnancy. However, some trisomic embryos (e.g. trisomy 21, trisomy 18, trisomy 13, and trisomy X) may survive to birth. Of the human trisomies that can yield live births, trisomy 13 generally exhibits the most severe birth defects.
Trisomy 13 is most often caused by random events during the division of egg or sperm cells (called meiosis), which can result in an egg or sperm cell with extra copies of chromosome 13. Random errors may also occur in the division of cells in the embryo (called mitosis), leading to a condition known as mosaic trisomy 13 or mosaic Patau. In mosaic individuals, some (but not all) cells in the body carry extra copies of chromosome 13. Like full trisomy 13, mosaic trisomy 13 is not considered to be inheritable.
In rare cases, a portion (or all) of chromosome 13 attaches, or translocates, to another chromosome, resulting in an inheritable form called translocation trisomy 13. Carriers of chromosome 13 translocations may have balanced translocations, in which the amount of genetic material from chromosome 13 is preserved. Therefore, these individuals may display no symptoms of trisomy 13 but have increased risks of producing children with excess amounts of chromosome 13.
Because of the high rate of spontaneous abortion and the poor prognosis for trisomy 13 patients, early prenatal diagnosis may be particularly important for prospective parents. Counseling is also important, especially for parents who carry chromosome 13 translocations, so that they may make informed decisions regarding the potential risks associated with carrying and caring for children with trisomy 13.
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