Well this is my theory but I'm pretty sure about this stuff, but once again it is all just educated reasoning.
Here goes, well we know that most of the time elemental stuff is genetic, hence the bloodlines having the same KG and stuff like Haku and his mom and the Uchiha having fire, and by using the first's DNA people can use his wood jutsu. So now that we have got the genetic part cleared up we also know that usually elemental affinities are thematic, which means that they go along with their villages theme.
Examples:
Hidden clouds- Lightning and some water users.
Hidden mist - water, Mizukage does mist KG's
Hidden Rock- earth element users,
Hidden sand - we only have like two elemental users we have seen so far, Temari and that guy that killed Hayate and they were both wind users. (Gaara's sand isn't an elemental technique)
Hidden Leaf - the only village that is really different but almost all of the people are able to use fire jutsu except for Naruto. examples- Uchiha, Kakashi, Jiraiya, Hiruzen
The genetics thing and the fact that village themes play into elemental affinities led me to this reasoning. Obviously Naruto is wind-natured and had to get it from one of his parents, well Kushina was from the Land of whirlpools which if we follow the theme then she would have had a water affinity which leaves Minato to have the wind affinity.
So there you go, that is my reasoning. For all of you who are going to argue for the sake of arguing don't just point out the fact that this hasn't been explicitly stated that Kushina uses water and Minato uses wind, try and be intelligent and find fault with my reasoning and logic instead.
