Genes, the fundamental units of heredity, carry the instructions for building and maintaining an organism. Each gene resides at a specific location on a chromosome and can exist in different versions, known as alleles. These alleles determine the various forms a trait can take, such as eye color or blood type. While many traits are governed by just two alleles for a given gene, some exhibit a more intricate inheritance pattern where multiple allele versions exist within a population.
What Multiple Alleles Are
Multiple alleles describe a situation where a gene has more than two possible allele forms within a population, though an individual organism, being diploid, can only carry two alleles for any given gene. The collective gene pool of a species, however, can harbor numerous variations. This means that while an individual might carry alleles ‘A’ and ‘B’, the population can have ‘C’ or ‘D’ for the same trait. The concept highlights the genetic diversity present across a group of organisms, rather than the limited number of alleles an individual can express.
This expanded set of allele options provides a richer genetic basis for variation compared to a simple two-allele system. While a two-allele system typically allows for only three genotypes and two phenotypes, multiple alleles substantially increase the number of potential genotypes and phenotypes. This leads to a wider range of observable characteristics and allows for greater adaptability and evolutionary potential within a population.
How Multiple Alleles Shape Traits
The presence of multiple alleles introduces more complex interactions that determine an organism’s observable traits, or phenotypes. In these systems, a hierarchy of dominance often exists among the different alleles. For example, one allele might be completely dominant over all others, while another is dominant only over a specific subset. This creates a linear order of expression, where the most dominant allele dictates the phenotype when present, followed by the next in the hierarchy if the top allele is absent.
Beyond simple dominance, multiple allele systems can also exhibit codominance, where both alleles in a heterozygous individual are fully and equally expressed without blending. Another interaction is incomplete dominance, where a heterozygous individual shows an intermediate phenotype that is a blend of the two alleles. These varied modes of interaction among multiple alleles contribute to the broad spectrum of traits observed in populations.
Common Examples of Multiple Alleles
One of the most widely recognized examples of multiple alleles in humans is the ABO blood group system. This system is governed by a single gene with three alleles: I^A, I^B, and i. The I^A allele results in the production of A antigens on red blood cells, while the I^B allele produces B antigens. The i allele is recessive and does not produce any antigens.
The interactions among these three alleles determine the four distinct blood types. Individuals with genotypes I^A I^A or I^A i have type A blood, and those with I^B I^B or I^B i have type B blood. The I^A and I^B alleles exhibit codominance, meaning that individuals with the genotype I^A I^B express both A and B antigens, resulting in type AB blood. Individuals with the genotype ii have type O blood because they produce neither A nor B antigens.
Another illustration of multiple alleles can be seen in the inheritance of coat color in rabbits. The gene responsible for coat color has four known alleles: C, c^ch, c^h, and c.
The C allele produces a full color coat and is dominant over all other alleles. The c^ch allele results in a chinchilla coat, which is dominant over the Himalayan and albino alleles. The c^h allele leads to a Himalayan coat, characterized by pigment only at the extremities, and is dominant over the albino allele. The c allele, which produces an albino rabbit, is recessive to all other alleles in this series.