What is co-dominate allele disorder and what do the parents' alleles have to be for the child to get it?

I really wanted to know what co-dominant allele disorder is because all of the online encyclopedias didn't have an answer for it. I know co-dominant is when both alleles are dominate and both are expressed, and that a disease is any change, other than an injury, that disrupts the normal function of the body, but i just can't seem to make the pieces fit.