OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/GFy_h8cu@10.57:4qg08nt-@8/Characteristics-and-Traits
Multiple Alleles
Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level, such that many combinations of two alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype in the natural population as the wild type (often abbreviated “+”). All other phenotypes or genotypes are considered variants (mutants) of this typical form, meaning they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.
An example of multiple alleles is the ABO blood-type system in humans. In this case, there are three alleles circulating in the population, each with a different type of signaling molecule on it called an antigen (Figure 14). The IA allele codes for A antigens on the red blood cells, the IB allele codes for B antigens on the surface of red blood cells, and the i allele codes for no antigens on the red blood cells. In this case, the IA and IB alleles are codominant with each other and are both dominant over the i allele. The human immune system makes antibodies for the types of antigens that a person does not have on their red blood cells. Antibodies are found in the watery component of the blood, which is called plasma. The antibodies allow the immune system to detect foreign blood entering the circulatory system and to respond by destroying the foreign blood.
Although there are three alleles present in a population, each individual only gets two of the alleles from their parents. This produces the genotypes and phenotypes shown in Figure 15. Notice that instead of three genotypes, there are six different genotypes when there are three alleles. The number of possible phenotypes depends on the dominance relationships between the three alleles.
Multiple Alleles Confer Drug Resistance in the Malaria Parasite
Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae, and is characteried by cyclic high fevers, chills, flu-like symptoms, and severe anemia. Plasmodium falciparum and P. vivax are the most common causative agents of malaria, and P. falciparum is the most deadly. When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1 percent. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.
References
Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.