Higher Biology - Phenotype ratios in a dihybrid cross

[_sophie_]

So i'm just a bit confused about the phenotype ratio's in a dihybrid cross and linked genes.
Not quite sure what i'm actually asking but if anyone can help please do! There is a question that comes up and it's when the phenotype ratio's aren't the usual ones what has happened to the genes? The answer is that they are linked but crossing over has occurred (is that right?) but then how do you know if the genes aren't linked? And how do you know if independant assortment has happened or does that always happen? I'm just so confused! Please make it clearer for me if you can! :)

[Calibax]

Not sure exactly what you're asking.
The normal ratio for a single trait would be 3:1 dominant:recessive
For two traits (dihybrid) the ratio would be 9:3:3:1 (for both dominant either dominant or both recessive). If this ratio is not found then independent assortment has not happened (as you say probably due to gene linkage)

[_sophie_]

Ohh ok so only linked genes have independant assortment (unless not the usual ratio?)
What about crossing over aswell?
Thanks though!