From my understanding, incomplete dominanace occurs when one allele results in a non-functional or no protein, and the other results in a functional protein. This answers my homework question; I simply don't understand how it works, and I generally like to understand my answers to questions.  If the protein isn't functional or isn't even there, wouldn't just one be expressed?

According to wikipedia, codominanace results in red + white = pink. According to my book that's incomplete dominance. Which one should I believe?

I kind of understand epistasis.

I guess I'm just wondering how exactly proteins function in the expression of genes. Next time I'll try to be more specific; my homework is done, these are questions generated after doing it. If you still can't answer, I understand. I don't want anyone to do my homework for me, and you shouldn't have to do homework for people!

Thanks for being so honest regarding homework I hope the info below helps - please do post again if not. Overall I agree with you that it is very complicated and somewhat confusing. That is because many of the terms used below to explain the inheritance patterns of things like plant colours happened long before modern molecular biology and genetics came along. The relationship therefore between the genetic terms and the molecular explanation at the protein levels is not always as simple as one might expect!

Incomplete dominance (sometimes called partial dominance) is a heterozygous genotype that creates an intermediate phenotype (a mixture). In this case, only one allele (usually the normal or "wild type") of a single gene is expressed in a doseage dependent manner, which results in an intermediate phenotype. A cross of two intermediate phenotypes will result in the reappearance of both parental phenotypes and the intermediate phenotype. The classic example of this is the colour of flowers ie you can get pink as well as red and white and another is skin colour in some animals. As to your question of how this relates to protein production and function, generally in incomplete dominance the wildtype allele makes a functional protein and the recessive one either makes none or it does not have the correct function. When you then breed 2 incomplete dominant alleles and if you think of this in terms of an enzyme giving 100%, 50% or 0% activity or the amount of red pigment, you can then start to see how you can get pink as well as red and white.

In codominance, neither phenotype is recessive. Instead, the heterozygous individual expresses both phenotypes. A common example is the ABO blood group system. The gene for blood types has three alleles: A, B, and i. i causes the O type and is recessive to both A and B. The A and B alleles are codominant with each other. When a person has both an A and a B allele, the person has type AB blood. Here A and B proteins are both made and are functional and both are expressed on the cells.

Epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. In incomplete dominace there clearly is epistasis whereas in codominance that may not always occur ie they are independent of each other and can co-exist.