Recombination mapping. (a) The procedure for analyzing a two-factor cross. Diploid cells and gametes are illustrated to show the origin of recombinant F₂ progeny. Chromosomes with dominant linked genes are purple; chromosomes with recessive linked genes are yellow. Crossing over results in purple-and-yellow hybrid chromosomes. F₁ progeny are crossed with a homozygous double-recessive fly to analyze the genotypes of gametes produced by the F₁ hybrid. (b) Analysis of linked genes using three two-factor crosses. Crossing over is indicated by x between two chromosomes. (c) Analysis of linked genes using three three-factor crosses. Two- and three-factor crosses lead to the same conclusion about gene order (ACB).