There are four basic kinds of blood types: A, B, O, and AB. Type A blood means that the marker proteins on the outside of the cell are in an the configuration for A blood, type b means the proteins are in the configuration for B blood, type O blood means there are no marker proteins, and type AB blood means that there are both type A blood marker proteins and type B blood marker proteins. Each blood type has its own gene makeup as well. There are three alleles for blood: Ia (type A), Ib (type B), and io (type O). The Ia and Ib alleles are co-dominant, and the io allele is recessive. Therefore you can get type A blood by having either a IaIa genotype or a Iaio genotype, and you can get type B blood with a IbIb or Ibio genotype. Type O blood can only be gotten by having the ioio genotype and type AB blood can only be gotten by having a IaIb genotype. Another thing about type O blood is that it is the "Universal donor", or can be given to anyone because it has no marker proteins, and type AB blood is the "Universal recipient", because it has both types of marker proteins and therefore accepts types A, B, O, and AB blood. As well as the ABO types of blood, there is the Rh factor which is determined by two alleles. These two alleles can be called D & d, with D being dominant and d being recessive. The Rh factor can cause problems during pregnancy if the mother is dd and the father is Dd or DD. The Rh factor is written by putting either a minus (if the person has the recessive trait) or a plus (if the person has the dominant trait.
Source(s):
http://anthro.palomar.edu/blood/Rh_system.htm

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