Background Codominance Problems (use for 10-13): Sickle cell anemia, as we have learned about before, is a genetic disorder that can be passed on to offspring. If a child receives the allele for normal red blood cells from either parent they will show the phenotype of normal red blood cells. If a child receives the allele for sickled red blood cells from either parent they will show the phenotype of sickle red blood cells. If they get one allele for sickle and one for normal, they will show both the phenotypes for sickle cells and normal red blood cells. In other words, alleles are never hidden, whatever is present, shows. Red blood cell type has been found to be linked with malaria resistance as shown below.