Interested to know how does a person inherit trisomy 13?

Genetic Mutation: Whenever you make germ cells (sperm and eggs) your body mixes up some of your genetic material and then separates it into discrete packets called chromosomes. You have 23 chromosomes in each germ cell. Sometimes in the process of making germ cells, the mechanism for sorting them out can get rusty (especially with age) and you can end up with an extra copy of number 13 -- t13.

They don't: Trisomy 13 is a spontaneous new event that occurs when 2 #13 chromosomes in either the egg (sperm) join with the one from the opposing side. The other 22 chromosomes pass only one of each. The fetus ends up with 47 chromosomes, not the normal 46. It is not passed on the genes like eye color or other features. It is an accident of nature.

Patau Syndrome: With Trisomy 18 instead of having 2 chromosome 18's, it appears 3 times. So instead of having 46 chromosomes, the individual has 47 chromosomes. This may occur in all cells or just a percentage of cells. This occurs by chance & is Not usually inherited.