Non-disjunction. The majority of cases of the lethal patau or trisomy 13 syndrome are caused by nondisjunction (47, +13) related to advanced maternal age. Robertsonian translocations of either parent can increase the risk for that couple, although most affected embryos die in utero.
Extra chromosome 13. Patau syndrome is a syndrome caused by a chromosomal abnormality, in which some or all of the cells of the body contain extra genetic material from chromosome 13. Symptoms include polydactyly, holoprosencephaly, coloboma of the iris, scalp defect, clenched, overlapping fingers, many midline defects, severe mental retardation, short life spans (<1yr avg)
It's not. Patau syndrome arises from a non-disjunction event prevents the 18 chromosome from dividing its pair & sends 2, not one to the egg. At conception the sperm brings along another 18 & there ends up being 3. The presence of the extra 18 causes the developing fetus to misread the genetic instructions and many defects occur.