I heard it actually shows up on the grandchildren. I've never bred with an NPC so I can't be certain, but I think it's been confirmed that the hidden traits skip a generation.

The heir in my legacy family has a baby with a firefighter. So far he doesn't have the pyromaniac trait so I will watch for it in his kids.

oooh...my sim-self married the pizza woman and their daughter is a young adult marrying a male made, So the will grand kid be a pizza lover? and does a maid bring any cool secret traits for my great grand kid ?

I have some news on the Pyromaniac trait - or lack thereof.

My sim, Brett (female) had a baby with Barry, a fireman (he only recently moved in so he was still a fireman when Brett got pregnant). Their son, Jamison, is now a teen and so far doesn't have a Pyromaniac trait BUT I did accidentally click on Jamison one day and one of the options that was available was "Burn Something Nearby". I tried to take of pic of it but it didn't work - I will do a Print Screen next time and post it. So Jamison walked downstairs into the office and set a chair on fire. I tried to get him to extinguish it but I couldn't select it and a minute later the fire just burnt out anyway. Here are some pictures:

Lighting the chair on fire.


The fire rages but no one cares - there's no screaming and no option to extinguish.


The chair, post-fire. It's still in perfect condition.


So maybe there isn't an actual trait, maybe they just get that option? I'll let you know if anything shows up in Jamison's kids.

I'll have to see. I've gotten a few maid children, but I can't honestly recall if they ever made a mess. When you tinker everything to self-clean and I had a sim whom cleaned as well, it's a hard thing to spot.

Darn fireman keeps giving me fines... the father and son have the trait, and I set of the fire alarm everytime I tell them to "Burn something near by"

The fireman's grandchildren (Jamison's kids) also have the "Burn Something Nearby" option. I wonder if it will carry on to every generation now?