are not necessarily all that readily distinguishable, and which term applies can depend on which specific phenotypic trait you are considering. An excellent example that you are probably familiar with is Tay-Sachs disease as discussed in Campbell and Reece (2005). Tay-Sachs disease is an inherited human disorder in which the brain cells are unable to metabolize certain lipids because a particular enzyme does not function correctly the accumulation of lipids in the brain cells causes seizures, blindness, degeneration of motor and mental skills, and ultimately results in death within a few years. Only children who are homozygotes for the Tay-Sachs allele have the disease, and so this allele qualifies as recessive at the organismal level. In heterozygotes, the activity level of the lipid-metabolizing enzyme is intermediate between that in children homozygous for the normal allele and that in children with Tay-Sachs disease. Thus, at this biochemical scale an intermediate phenotype is observed that is characteristic of incomplete dominance of either allele. Finally, heterozygotes are found to produce equal numbers of normal and dysfunctional enzyme molecules, and so the normal and Tay-Sachs alleles appear codominant at the molecular level.

Steve
www.BoaMorph.com

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