One of Mendel’s important contributions to the study of heredity is the concept of dominance—the idea that although an individual organism possesses two different alleles for a characteristic, the trait encoded by only one of the alleles is observed in the phenotype. With dominance, the heterozygote possesses the same phenotype as one of the homozygotes.

Mendel observed dominance in all the traits he chose to study extensively, but he was aware that not all characteristics exhibit dominance. He conducted some studies of the length of time that pea plants take to flower. When he crossed two homozygous varieties that differed in their flowering time by an average of 20 days, the length of time taken by the F₁ plants to flower was intermediate between those of the two parents. When the heterozygote has a phenotype intermediate between the phenotypes of the two homozygotes, the trait is said to display incomplete dominance.

COMPLETE AND INCOMPLETE DOMINANCE Dominance can be understood in regard to how the phenotype of the heterozygote relates to the phenotypes of the two homozygotes. In the example presented in the upper panel of Figure 4.13, potential flower color ranges from red to white. One homozygous genotype, A¹A¹, produces red pigment, resulting in red flowers; another, A²A², produces no pigment, resulting in white flowers. Where the heterozygote falls in the range of phenotypes determines the type of dominance. If the heterozygote (A¹A²) produces the same amount of pigment as the A¹A¹ homozygote, resulting in red flowers, then the A¹ allele displays complete dominance over the A² allele; that is, red is dominant over white. If, on the other hand, the heterozygote produces no pigment, resulting in flowers with the same color as the A²A² homozygote (white), then the A² allele is completely dominant, and white is dominant over red.

When the phenotype of the heterozygote falls in between the phenotypes of the two homozygotes, dominance is incomplete (see the bottom panel of Figure 4.13). With incomplete dominance, the heterozygote need not be exactly intermediate (pink in our example) between the two homozygotes; it might be a slightly lighter shade of red or a slightly pink shade of white. As long as the heterozygote’s phenotype can be differentiated and falls within the range between the two homozygotes, dominance is incomplete. The important thing to remember about dominance is that it affects the way that genes are expressed (the phenotype), but not the way that genes are inherited.

CODOMINANCE Another type of interaction between alleles is codominance, in which the phenotype of the heterozygote is not intermediate between the phenotypes of the homozygotes; rather, the heterozygote simultaneously expresses the phenotypes of both homozygotes. An example of codominance is seen in the MN blood types of humans.

The MN locus encodes one of the types of antigens on red blood cells. Unlike antigens of the ABO and Rh blood groups (which also encode red-blood-cell antigens), MN antigens do not elicit a strong immunological reaction, and therefore MN blood types are not routinely considered in blood transfusions. At the MN locus, there are two alleles: the L^M allele, which encodes the M antigen; and the L^N allele, which encodes the N antigen. Homozygotes with genotype L^ML^M express the M antigen on their red blood cells and have the M blood type. Homozygotes with genotype L^NL^N express the N antigen and have the N blood type. Heterozygotes with genotype L^ML^N exhibit codominance and express both the M and the N antigens; they have blood type MN.

Some students might ask why the pink flowers illustrated in Figure 4.13 exhibit incomplete dominance: Why is this trait not an example of codominance? In this situation, the heterozygote is not producing both red and white pigments, which then combine to produce a pink phenotype. The heterozygote produces only red pigment, but the amount produced by the heterozygote is less than the amount produced by the A¹A¹ homozygote. So here, the alleles clearly exhibit incomplete dominance, not codominance. The differences between complete dominance, incomplete dominance, and codominance are summarized in Table 4.4. TRY PROBLEM 26

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LEVEL OF PHENOTYPE OBSERVED MAY AFFECT DOMINANCE Many phenotypes can be observed at several different levels, including the anatomical level, the physiological level, and the molecular level. The type of dominance exhibited by a characteristic depends on the level of the phenotype examined. This dependency is seen in cystic fibrosis, a common genetic disorder in Caucasians that is usually considered to be a recessive disease. People who have cystic fibrosis produce large quantities of thick, sticky mucus, which plugs up the airways of the lungs and clogs the ducts leading from the pancreas to the intestine, causing frequent respiratory infections and digestive problems. Even with medical treatment, patients with cystic fibrosis suffer chronic, life-threatening medical problems.

The gene responsible for cystic fibrosis resides on the long arm of chromosome 7. It encodes a protein termed cystic fibrosis transmembrane conductance regulator (CFTR), which acts as a gated channel in the cell membrane and regulates the movement of chloride ions into and out of the cell. Persons with cystic fibrosis have a mutated, dysfunctional form of CFTR that causes the channel to stay closed, so chloride ions build up in the cell. This buildup causes the formation of thick mucus and produces the symptoms of the disease.

Most people have two copies of the normal allele for CFTR and produce only functional CFTR protein. Those with cystic fibrosis possess two copies of the mutated CFTR allele and produce only the defective CFTR protein. Heterozygotes, who have one normal and one defective CFTR allele, produce both functional and defective CFTR protein. Thus, at the molecular level, the alleles for normal and defective CFTR are codominant because both alleles are expressed in the heterozygote. However, because one functional allele produces enough functional CFTR protein to allow normal chloride-ion transport, heterozygotes exhibit no adverse effects, and the mutated CFTR allele appears to be recessive at the physiological level. The type of dominance expressed by an allele, as illustrated in this example, is a function of the phenotypic aspect of the allele that is observed.

CHARACTERISTICS OF DOMINANCE Several important characteristics of dominance should be emphasized. First, dominance is a result of interactions between genes at the same locus (allelic genes); in other words, dominance is allelic interaction. Second, dominance does not alter the way in which the genes are inherited; it influences only the way in which they are expressed as a phenotype. The allelic interaction that characterizes dominance is therefore interaction between the products of the genes. Finally, dominance is frequently “in the eye of the beholder,” meaning that the classification of dominance depends on the level at which the phenotype is observed. As seen for cystic fibrosis, an allele may exhibit codominance at one level and be recessive at another level.