Four homozygous recessive mutant lines of Drosophila melanogaster (labelled 1 through 4) showed abnormal leg coordination, which made their walking highly erratic. These lines were inter-crossed; the phenotypes of the F1 flies are shown in the following grid, in which + represents wild-type walking and How many different types of genes were mutated in creating these four lines? A. 3 genes because all crosses except 2x3 or 3x 2 complement. B. 2 genes because lines 2 and 3 carry alleles of two different genes C. Two strains carry mutant alleles of 4 separate genes D. 3 genes because crosses 2 x 3 and 3 x 2 show wild type walking E. 4 genes because 1 x 1, 2 x 2, 3 x 3, and 4 x 4 fail to complement