Final answer:
Codominance is a form of genetic inheritance where both alleles from each parent are distinctly and equally expressed in the offspring, such as in the ABO blood group system, and the MN blood group in humans.
Step-by-step explanation:
In codominance, both alleles from each parent are equally and distinctly expressed in the offspring. Unlike complete dominance, where one allele completely masks another, in codominance, the traits of both alleles are visible in the phenotype. For instance, the ABO blood group system showcases codominance. Individuals who carry both A and B alleles have type AB blood, with both type A and type B antigens expressed on the surface of their red blood cells.
Codominance also signifies that no single allele is dominant over the other; instead, both contribute to the phenotype. This concept also extends to scenarios where multiple alleles exist within a population level, although an individual can have only two alleles for a gene.
Another classic example of codominance is the MN blood group in humans, where both M and N alleles are expressed equally in heterozygotes. Thus, codominance represents an important variation of inheritance that diverges from the classical Mendelian dominant-recessive framework.