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Multiple Alleles and Codominance
In humans and other organisms, there are traits that have three or more different types of alleles (genes). Whenever a given trait has three or more different alleles, we say that the trait has multiple alleles. One example of a trait the has multiple alleles is the human ABO blood group trait. There are three different alleles - allele A, allele B and allele i. If the chromosome contains the allele A, then protein A is created and the red blood cells of that particular individual will contain protein A on their membrane. If the chromosome contains the allele B, then protein B will be created and the red blood cells will have protein B on their membrane. Finally, if the chromosome contains allele i, then neither protein A nor protein B will be synthesized. These three alleles that constitute the ABO blood group trait observe a pattern of inheritance called codominance (co-dominance). In co-dominance, neither of the alleles are dominant with respect to the other and a heterozygous individual will express both phenotypes. That is, if the individual contains allele A on one chromosome and allele B on the second homologous chromosome, then both proteins will be expressed and the red blood cells will contain both protein A and protein B on their cell membrane. There are four different types of phenotypes - blood group A, blood group B, blood group AB and blood group O.
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