Three different types of alleles determine an individual's blood type: A, B, and O.
Alleles are one of the many genetic information found in the chromosomes of our DNA and by extension, in our blood. Each one of us inherit one allele from our father and another allele from our mother. Put these two alleles together and what we have is our genotype.
To better explain this:

Parent 1 Parent 2 Genotype Offspring's
Blood Type
A A A+A A
A B A+B AB
(B A B+A AB)
B B B+B B
B O B+O B
O O O+O O

A and B alleles are dominant while O is recessive. An A allele from one parent and an O allele from another will result in a type A blood baby. The same is true for a B allele and an O allele pair; a type B blood baby. A type O blood means that there are no A or B allele in the genotype. A type AB parent can pass on either the A or the B allele whereas a type O parent can only pass on the O allele.

If you have type AB blood, you can receive blood transfusions from types A, B and O. Type O can give blood to all types but can only receive its own type.

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