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About 30% of the genes in humans are di-allelic, that is they exist in two forms.
About 70% are mono-allelic, they only exist in one form and they show no variation.
A very few are poly-allelic having more than two forms.
Poly-allelic genes are usually associated with tissue types. They are so varied that they provide us with our genetic finger print.
This is very important to our immune system which must tell the difference between our own cells and invading disease causing microbes.
The ABO blood system
This is a controlled by a tri-allelic gene. It can generate 6 genotypes.
The alleles control the production of antigens on the surface of the red blood cells. Two of the alleles are co-dominant to one another and both are dominant over the third.
Blood types A and B have two possible genotypes - homozygous and heterozygous.
Blood types AB and O only have one genotype each.
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