because from AABb we got four gametes AB AB Ab Ab , and similarly from aaBb we got four gametes aB aB ab ab and then if we do a cross between this gametes by using punnet square we get 0(zero) -number of AABB individuals, 4 - AaBB , 8 - AaBb , and 4 - Aabb individuals respectively. then by simplifying this we get the ratio 0:1:2:1.