First explain a little about human ABO blood groupings to your students, including the fact that types A and B are due to genes coding for protein markers on the surfaces of red blood cells, and that A and B are codominant. Type O, which indicates an absence of either type A or B surface protein markers, is recessive to the other two.
Then pose a paternity lawsuit problem, such as the one that follows. (Admittedly, with the advent of DNA fingerprinting, such a suit is archaic, but it still serves to entice students into thinking about genetics problems.)
A woman whose blood type is O has a baby with blood type B. She is accusing a man, named Sam, of fathering her child, and wishes to receive child support from him. Sam's blood type is AB. Could he have fathered this child? (Yes.) Can you prove that he is the father of this child? (No, not conclusively.)
In Mendel's pea plants, purple flower color is dominant to white, and green pod color is dominant to yellow pod color. Have students cross a pea plant that is homozygous dominant for both traits with one that is homozygous recessive (PPGG x ppgg). All F1's will be heterozygous for both traits, and all will show both dominant phenotypes.
Next, have students cross two F1's using a 4 x 4 Punnett square. They should come up with a phenotypic ratio of 9:3:3:1.