Is it true that trisomy 13 caused by inbreeding?

Not true: Trisomy 13 (3 copies of chromosome 13 in all your cells) is caused by an error in the way one of your two chromosome 13s move when you make an egg or a sperm. It's called nondisjunction. When it happens, it's no one's fault. Marriage between close relatives, like 1st cousins, is common in many places in the world, and trisomy 13 is just as rare there as it is everywhere else.

It doesn't: Trisomy 13 is a spontaneous new event that occurs when 2 #13 chromosomes in the egg ( or sperm) join with the one from the opposing side. The other 22 chromosomes pass only one of each & form pairs. The fetus ends up with 47 chromosomes, not the normal 46. It is an accident of nature. Inbreeding increases risk of some metabolic issues, but not trisomy,

Patau's syndrome: Inbreeding does not increase risk for Trisomy 13.