As soon as you introduce problems with more than one gene, you also introduce the possibility of interactions between genes. Epistasis is the interference of one gene with the expression of a totally different gene. This isn't something you will be expected to deal with in problems, but it is interesting to know about. The classic example of an epistatic condition is albinism. This gene is one of those which controls the production of a molecule called melanin, which is the primary pigment molecule in human skin, hair and eye color.
Melanin is brown in color.
Yes, blue eyes are produced by a brown pigment. Albinos have pink-white skin and red eyes. The color in the eyes comes from the highly vascularized retina in the back of the eyeball. There is no pigmentation in the iris of the eye, where the melanin which makes blue or brown eyes is deposited. If we use the same example we followed above, but substitute the albino gene for the earlobe gene, we can see the effect that epistasis has on the phenotypic ratio.