Misleading students into thinking their parents can't be their bio-parents seems like a really important reason not to use fake examples of Mendelian traits.
BTW the technology exists (and have been sold for nearly 20 years) to determine paternity, sex and genetic diseases in an infant 2-6 weeks after CONCEPTION from the mother's blood.
The same applies to other cases where parents aren't genetically related to their children (i.e., donor egg and/or sperm).
[1] https://www.theatlantic.com/family/archive/2019/07/adoption-...
Whether or not the parents had a paternity test performed wouldn't have any bearing on it.