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Multiple Alleles

Problems
Multiple alleles
Monohybrid
Mendel
Incomplete
Disorders
Dihybrid
Codominance
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.