Is trisomy 13 dominant or recessive?

Neither: Any defect like trisomy 13, that involves acquiring an extra chromosome, has nothing to do with dominant or recessive genes. It occurs when usually the egg carries 2 rather than 1 of the 13 chromosomes. Fertilization by a normal sperm with one of every chromosome creates an embryo with three # 13's.If the embryo survives pregnancy it emerges as a trisomy 13 infant with all the problems they have.

Trisomy 18: Trisomy 13 or Patau Syndrome is not related to dominant or recessive genes. With Trisomy 18 instead of having 2 chromosome 18's, it appears 3 times. This may occur in all cells or just a percentage of cells. This occurs by change & is Not usually inherited.