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.