Thus, lipids accumulate in the brain, causing the disease symptoms and ultimately leading to death. At the organismal level , since heterozygotes are symptom free, it appears that the normal allele is completely dominant and the Tay-Sachs allele is recessive. At the biochemical level , inheritance of Tay-Sachs seems to be incomplete dominance of the normal allele, since there is an intermediate phenotype. Heterozygotes have an enzyme activity level that is intermediate between individuals homozygous for the normal allele and individuals with Tay-Sachs Disease. At the molecular level , the normal allele and the Tay-Sachs allele are actually codominant.
Heterozygotes produce equal numbers of normal and dysfunctional enzymes.
They lack disease symptoms, because half the normal amount of functional enzyme is sufficient to prevent lipid accumulation in the brain. In other words, dominant alleles are not necessarily more common and recessive alleles more rare. The inheritance of the ABO blood group is an example of a locus with three alleles. I A and I B are codominant since both are expressed in heterozygotes. I A and I B are dominant to allele i, which is recessive.