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Severe Chromosomal Disorders: Trisomy 13 and 18

written by: •edited by: lrohner•updated: 2/28/2011

Chromosomal disorders are rare but devastating. What two chromosomal errors are the worst? This article will outline the two which are the most injurious, and oftentimes fatal, to individuals.

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    Chromosomes and Chromosomal Abnormalities

    Roughly one in 150 babies in the United States are born with a chromosomal abnormality, according to the March of Dimes. Such abnormalities can result in miscarriage, stillbirth or severe physical and mental defects. To comprehend the severity of these types of abnormalities, one must first understand the function of chromosomes.

    Chromosomes are located within every cell of the body and are critical for the body’s survival because they house a person’s genes. There are roughly 20,000 to 25,000 genes in the body. Genes determine traits such eye color, but they also control crucial developmental functions of the body. Humans have a total of 46 chromosomes, 23 pairs each — one pair inherited by one’s mother and the other by one’s father.

    Chromosomal disorders result from errors in either the structure or number of chromosomes present in a person. These types of abnormalities occur when there is an error in the development of the sperm or egg before fertilization. While the most common error consists of too few or too many chromosomes, deletions, translocations, inversions or duplications of a chromosome are also a possibility. When an abnormal egg or sperm unites with its normal counterpart, the embryo will end up inheriting that abnormality.

    While chromosomal disorders vary in severity, depending on the genes affected, there are a couple of chromosomal abnormalities that oftentimes cause fatality before the baby’s first birthday due to life-threatening complications. What two chromosomal errors are the worst? These disorders are known as trisomy 13 and trisomy 18.

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    Trisomy 13

    Most cases of trisomy 13, also known as Patau syndrome, result from having three copies of chromosome 13 in each cell instead of two. Nonetheless, trisomy 13 can also occur when a segment of chromosome 13 attaches to a different chromosome during the formation of the reproductive cell (sperm or egg) or during fetal development. There is also what is known as mosaic trisomy 13, which is the condition where the extra chromosome is located in some but not all cells.

    Trisomy 13 is associated with severe mental and physical disability. Individuals born with trisomy 13 will often have heart defects, poorly developed eyes (microphthalmia), extra appendages (fingers or toes), a cleft lip or palate, and weak muscles (hyptonia). Since some these symptoms are life-threatening, many infants born with this disorder will die within the first days to weeks of life and only 5 to 10 percent will live to celebrate their first birthday.

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    Trisomy 18

    Trisomy 18, also known as Edwards syndrome, is similar to trisomy 13 in that the most common occurrence of it is from an extra copy of chromosome 18. It can come about, however, when a segment of chromosome 18 attaches to a different chromosome. Like many other chromosomal disorders, the abnormality may present itself in some but not all cells. This is rare, however.

    Typical characteristics of an individual born with trisomy 18 are heart defects, kidney problems, rocker bottom feet, the intestinal tract outside of the stomach (omphalocele), an umbilical hernia, small head (microcephaly), pocket of fluid on the brain and severe developmental delays. Due to the fact that some of these are life-threatening, 50% of babies carried to term will be stillborn. Even if the baby is alive upon birth, less than 10 percent of babies born will survive to celebrate their first birthday.

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    Prevention, Awareness, and Treatment

    Roughly one out of 16,000 newborns are born with trisomy 13 while 1 out of 5,000 newborns are born with trisomy 18.

    There is currently no cure for chromosomal disorders since they are the result of random genetic mutations. While this is the case, a pregnant woman may have genetic screening performed to understand the risk of conceiving a child with a chromosomal abnormality such as trisomy 13 or 18.

    Even though both of these disorders are often fatal, there are people who live well into their twenties and thirties with these respective conditions.There are also treatments out, but they are done on a case by case basis since it needs to be taken into account the exact genes affected by the abnormality.

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    Support Organization for Trisomy Disorders: Trisomy 13 and 18

    KidsHealth from Nemours: Genetic Testing

    PubMed: Edwards Syndrome

    PubMed: Patau Syndrome