ABO blood group
The ABO system is the most famous everyday example of human genetics in action: it determines whether your red blood cells carry the A sugar marker, the B marker, both, or neither — giving the familiar blood types A, B, AB, and O. It is governed by a single gene with three common alleles.
The three alleles are usually written I-A, I-B, and i. The I-A and I-B alleles each add a different sugar to the cell surface and are codominant with each other, so a person with both makes both markers and is type AB. The i allele adds nothing and is recessive to both, so type O arises only when someone inherits two i alleles.
This neatly combines two ideas: multiple alleles (three versions of one gene in the population) and codominance (A and B expressed together in AB individuals). The system matters in transfusion because the immune system attacks any A or B marker it does not recognize as its own — which is why blood types must be matched before a transfusion.
Two type-A parents can have a type-O child if each carries a hidden i allele: each is genotype I-A i, and the child inherits i from both.
Hidden recessive i alleles let two type-A parents produce a type-O child.
A and B are codominant, while O is recessive to both — so AB shows both markers, but O appears only with two copies of the i allele. The separate Rh system adds the familiar plus or minus.