Genetics, sickle cell anemia
Whether alleles are "codominant" or "recessive" or "dominant" depends on the level at which you look at it.

If you look at it from the organismal "healthy vs sick" point of view, then sickle cell anaemia is "recessive" because the individual is only sick when they have both sickle cell alleles.

If you look at it from again the organismal point of view, but this time looking at the trait as well, then you find that (if we say "A" is the anaemic allele and "N" is the normal allele) AN has a different trait (called sickle cell trait) than NN, so you could say the alleles are codominant because both the A and the N "do something" in the heterozygote.

If you look at the molecular level, again you will find that sickle cell anaemia is codominant. The gene codes for the haemoglobin beta gene. So if you have AN, then one of your alleles will be coding for the defective haemoglobin, and the other will be coding for the normal one. So you would have 2 types of haemoglobin in your blood, which means that it is "codominant."