The correct answer is: Blood type in humans.
The blood type in humans or ABO blood type is controlled by a single gene (the ABO gene) with three types of alleles (multiple alleles), a, b, i. a and b alleles express a special dominance relationship called codominance, meaning that they together form AB blood type. A is dominant over i so they together give A blood type. B is is also dominant over I, giving the B blood type.