Explain codominance in terms of the ability or inability of the alleles on single locus to make a product.?

For some traits, two alleles can be codominant. That is to say, both are expressed in heterozygous individuals. An example of this is people who have an AB blood type for the ABO blood system. When they are tested, these individuals actually have the characteristics of both type A and type B blood. Their phenotype is not intermediate between the two