As you have determined, the f1 is heterozygous for four loci that affect fruit segment number (a+ a 0 b+ b 0 c+ c 0 d+ d 0). This means that the f1 can produce 16 distinct gamete types, which contain different numbers of additive alleles (designated with the "+" superscript). Sort the 16 gamete types into bins based on the number of additive alleles they contain.