Which trait is normally controlled by multiple alleles but not multiple genes? Blood type in humans birth weight in calves eye color in humans length of ears in corn plants

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.