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The Type of Mutation that Causes Patau Syndrome

written by: •edited by: Paul Arnold•updated: 12/4/2009

Patau syndrome is a severe chromosomal abnormality that is primarily caused by the addition of an extra copy of chromosome 13. It means that most cells in the body will have three copies of this chromosome, instead of the usual two.

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    How is Patau Syndrome Inherited?

    Patau syndrome (also known as trisomy 13) is a congenital condition, that is, it is present at birth. The survival rates are low. Many infants born with the condition die within a few days. Its rate of incidence tends to increase with the age of the mother. Those children who do survive tend to have serious physical and mental impairments. In most cases the condition is not inherited, it is usually caused by a spontaneous mutation that occurs during the formation of sperm and eggs.

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    Patau Syndrome and Chromosome Abnormalities

    The type of mutation that causes Patau syndrome takes place during meiosis when either the sperm or egg cells are being created. The names given to these mutations are nondisjunction of chromosomes and Robertsonian translocation.

    There are 46 chromosomes in each cell, apart from red blood cells and the sex cells. Those 46 chromosomes are 23 pairs, numbered 1-22 and then either an X and Y (boy) or an X and another X (girl).

    In nondisjunction the chromosome 13 pair fails to come apart during the cell division (meiosis) that leads to the formation of either an egg cell or a sperm cell. So when the parent cell is cleaved instead of there just being one chromosome 13 in a daughter gamete cell there are two. When this combines with another gamete, that has the usual complement of one chromosome 13, the result is a zygote with three copies of the chromosome.

    It is not well understood at the present moment in time just how this additional genetic material causes the disruption that leads to the onset of the condition.

    The other type of mutation that causes Patau syndrome is a Robertsonian translocation. Here a copy or partial copy of chromosome 13 randomly attaches itself to another chromosome.

    Sometimes, a translocation occurs and there’s no additional genetic material in the cell. The portion of chromosome 13 that is translocated has merely swapped placed with a portion of another chromosome. Where this occurs and there is no net gain or loss of genetic material, the individual usually has no problems related to Patau. However, people with these balanced translocations are at a higher risk of giving birth to a child with Patau syndrome.

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    Mosaic Patau

    There is another form of the chromosomal abnormality - Mosaic Patau. Here, some cells will possess the extra chromosome and others will have their normal complement. The severity of physical ailments this causes is usually milder and dependent on the number and type of cells affected.