How to spyOn a global function and return a value in Jasmine

How can I spy on MyFunction and return 2 in Jasmine 2.0?

I have the following code that errors on the first line inside shouldPass with:

Error: function MyFunction(){ return 1; }() method does not exist

It appears to be using the entire function as the function name

MyFile.js:

MyFunctionToTest = function(){
    return MyFunction() + 1;
}

function MyFunction(){ return 1; }

MyFileSpec.js:

describe("myTest", function(){
    it("shouldPass", function(){
        spyOn("MyFile", MyFunction).and.returnValue(2);

        expect(MyFunctionToTest()).toEqual(3)
    })
})