Just as multiple genes can influence one trait, some individual genes can influence multiple, unrelated traits, a phenomenon called pleiotropy. In fact, this may be true of nearly all genes. Consider sickle-cell disease (also called sickle-cell anemia), a potentially fatal condition in which individuals produce defective red blood cells that change their shape, becoming sickle-shaped, when they lose the oxygen they carry. The defective blood cells can’t effectively transport oxygen to tissues, and they accumulate in blood vessels, causing extreme pain. Individuals with sickle-cell disease suffer shortness of breath and numerous other problems that lead to a significantly reduced life span. The gene responsible for sickle-cell disease encodes the molecule hemoglobin, the oxygen-carrying molecule in red blood cells: Hb^A is the allele for normal hemoglobin, and Hb^S is the abnormal, “sickle-cell” allele. Sickle-cell anemia occurs in individuals homozygous for the sickle-cell allele, Hb^SHb^S, but not in individuals carrying at least one copy of the normal allele, Hb^A (although heterozygous individuals produce both normal and sickling red blood cells, just not enough to cause sickle-cell anemia).

This hemoglobin gene is pleiotropic because, although it is just one gene, it causes a cascade of different phenotypic effects. Individuals homozygous or heterozygous for the sickle-cell allele, Hb^SHb^S or Hb^AHb^S, have abnormal hemoglobin, red blood cell deformation, and circulatory problems. Moreover, individuals with these genotypes also are resistant to the parasite that causes malaria. The resistance is due to the fact that the malarial parasite—which lives in red blood cells—cannot survive well in cells that carry the defective version of the hemoglobin gene. And because even individuals who are heterozygous for the sickle-cell allele have a significant number of sickling red blood cells, their bloodstream is just not a hospitable environment for the malarial parasite (FIGURE 7-24). Consequently, this one gene influences multiple traits.

Figure 7-24From one gene, multiple traits. The allele for sickle-cell disease is pleiotropic: it causes red blood cells to form an unusual, sickled shape, and it also provides resistance to malaria.

Another example of pleiotropy is the SRY gene. Named for “sex-determining region on the Y chromosome,” this gene causes fetal gonads to develop as testes shortly after fertilization. Following the testes’ secretion of testosterone, a cascade of other developmental changes then takes place, such as development of the internal and external male reproductive structures (including the prostate gland, seminal vesicles, vas deferens, penis, and scrotum). Ultimately, the SRY gene is responsible for numerous behavioral characteristics as well. These characteristics are described further in Chapter 9.