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Multiple Alleles

Multiple alleles
Chapter 7main
In some genetic systems there are more than one pair of alleles.  Also the genetic system doe not always reflect a pure dominance/ recessive relationship.  A case in point would be the ABO blood groups seen in Humans. 

In humans there are three alleles that determine blood types.  They are A, B and O alleles.  In this system, A is dominant to O and B is dominant to O.  But, A is not dominant to B or vice versa.  An individual with Blood type A can be AA or AO, A person with type B blood can be either BB or BO.  But when A and B alleles are seen together a third phenotype is seen.  That being AB type blood.  This is incomplete dominance.  There is also a fourth phenotype possible, that being type O blood which can only occur if the genotype is homozygous O.