What do the letters outside the punnet squares stand for ?

the atomic number of magnesium is 12. its nucleus must contain 12 protons and no electrons. the atomic number of an atom is determined by the number of protons.

magnesium is an electrolyte injection commonly used to treat low magnesium levels in your blood. it is also used to prevent or control seizures in women with preeclampsia or eclampsia.

on island x the species needed to adapt because they were being subject to predators which didn't inhabit island y.