Multiple Alleles
Certain traits do not fit into the two allele category, which creates a more complicated system then previously mentioned. The genetic trait in human beings that is most often used as an illustration of this is blood type.
For human beings, there are three different blood alleles. The first is I^A, which is responsible for the production of the A antigen. Likewise, I^Bis the allele responsible for production of the B antigen. While the first two have been dominant, the third i, which produces no antigens, is recessive. Combinations of these three alleles form the four distinct blood types: A, B, AB and O. The following chart shows these combinations.
Genotype Antigens produced Phenotype
I^A- I^A		A			A
I^A-i		A			A
I^B-I^B		B			B
I^A-i		B			B
I^A- I^B		A and B			AB
i-i		none			O
Here is a Punnet square concerning blood types:
			I^A	I^B
	I^A		I^A- I^A	I^A- I^B
	i		I^A-i	I^B- i
From this match, a person with A blood and a person with AB blood would have:
- A 50% chance of having a child with A blood
- A 25% chance of having a child with AB blood
- A 25% chance of having a child with B blood
Rh factor
Rh is a blood factor named after the Rhesus monkey. Problems can arise with the Rh factor if a father is positive and a mother is negative. Since it is a dominant trait, an Rh positive child can be in the uterus of an Rh negative mother. If this is the case, the mothers body will start to build up antibodies to combat the factor. This has no effect on the first child, but serious harm could be had by the second if the antibodies were present. Luckily, there is a vaccine that allows the fetu to grow in the mother, without developing Rh factor disease.
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