7.9–7.15: How are genotypes translated into phenotypes?

Phenotype diversity—which is all around us—has multiple sources.

As Mendel saw it, the world of genetics was straightforward and simple. We should be so lucky. Each of the traits he studied were coded for by a single gene with two alleles—one completely dominant and one recessive—and with no environmental effects. This, however, doesn’t capture the complexity of the world beyond Mendel’s pea plants. So, in this and the following sections we build up a more complex model of how genes influence the building of bodies.

We begin with the observation that the phenotype of heterozygous individuals sometimes differs from that of either of the homozygotes, and instead reflects the influence of both alleles rather than a clearly dominant allele.

One situation in which complete dominance is not observed is called incomplete dominance, in which the phenotype of a heterozygote is intermediate between the phenotypes of the two homozygotes. An example of incomplete dominance we can easily observe is the flower color of snapdragons (FIGURE 7-18).

Figure 7.18: Pink snapdragons demonstrate incomplete dominance. When true-breeding white and red snapdragons are crossed, offspring have pink flowers.

We can obtain true-breeding (homozygous) lines of snapdragons with red flowers and true-breeding (homozygous) lines that produce only white flowers. When plants from these two populations are crossed, we would expect—if one allele were dominant over the other—either all red or all white flowers. Instead, such crosses always produce plants with pink flowers. Interestingly, when we cross two plants with pink flowers, we get ¼ red-flowered plants, ½ pink-flowered plants, and ¼ white-flowered plants.

How can we interpret this cross? It seems that the plants with white flowers have the genotype C^WC^W and produce no pigment. At the other extreme, the plants with red flowers have the genotype C^RC^R and produce a great deal of pigment. The letter “C” refers to the fact that the gene codes for color, and the superscript “W” or “R” refers to an allele producing no pigment (white) or red pigment. We use these designations for the genotypes because it isn’t clear that either white or red is dominant over the other, and so neither should be represented by uppercase or lowercase. The pink flowers receive one of the pigment-producing C^R alleles and one of the no-pigment-producing C^W alleles, and so produce an intermediate amount of pigment. Ultimately, the intensity of pigmentation just depends on the amount of pigment chemical that is made by the flower-color genes.

An example of incomplete dominance in humans can be seen in the processing of cholesterol in the bloodstream. There is a plasma membrane receptor that allows cells (chiefly those in the liver) to remove cholesterol from the bloodstream (see Section 3-11), and the gene that codes for this receptor exhibits incomplete dominance. Individuals who carry two copies of a mutant allele (called FH) for this gene produce few (or even no) LDL receptors. In these individuals, circulating cholesterol levels are high and cardiovascular disease develops at a very young age. Individuals carrying one FH allele and one allele that codes for normal-functioning LDL receptors have significantly reduced levels of circulating cholesterol and a correspondingly lower risk of cardiovascular disease. Individuals carrying two copies of the allele for normal-functioning LDL receptors produce twice as many LDL receptors as heterozygotes and experience significantly lower levels of circulating cholesterol and a much lower risk of cardiovascular disease.

A second situation in which complete dominance is not observed is called codominance, in which the heterozygote displays characteristics of both homozygotes, playfully represented in FIGURE 7-19 (although “shirt phenotype,” of course, has no genetic basis). In codominance, neither allele masks the effect of the other. An actual example of codominance occurs with feather color in chickens. When white chickens are crossed with black chickens, all the offspring have both white and black feathers.

Figure 7.19: With codominance, a heterozygous individual shows features of both alleles.