Edit: usually. As for nuclear DNA, keep in mind all of the weird crossover failures that are rather common in a large population. These typically result in gene dosing problems and can lead to fetus inviability.

Then consider the epigenome, selected immune recognition, and microbiome. These can be/are environmental.

50/50 is the textbook story we teach before undergrad. It's easier to learn and still conveys the central idea of the model.