@MKSizzle For "play with genetics," the sim just needs to have been born. (Or maybe be a toddler?) You get an interface where you can pick the genetic donors, hit the teen or YA button so you can see the kid's grown-up appearance, and then keep clicking until you get a result you like. The only unaffected characteristics are the traits.
You know what? As usual, @igazor wrote up a better description than I would be able to produce:
https://forums.thesims.com/en_US/discussion/comment/16331665/#Comment_16331665
"Reroll genetic seed value" is used on a pregnant mother. It rerolls the entire outcome of the pregnancy, from how many babies and their genders to looks, to traits if you don't get to pick them yourself.
